Power storage device

ABSTRACT

A power storage device which has high charge/discharge capacity and less deterioration in battery characteristics due to charge/discharge and can perform charge/discharge at high speed is provided. A power storage device includes a negative electrode. The negative electrode includes a current collector and an active material layer provided over the current collector. The active material layer includes a plurality of protrusions protruding from the current collector and a graphene provided over the plurality of protrusions. Axes of the plurality of protrusions are oriented in the same direction. A common portion may be provided between the current collector and the plurality of protrusions.

TECHNICAL FIELD

The present invention relates to a power storage device and a method formanufacturing the power storage device.

BACKGROUND ART

In recent years, power storage devices such as lithium-ion secondarybatteries, lithium-ion capacitors, and air cells have been developed.

An electrode for the power storage device is manufactured by providingan active material over a surface of a current collector. As a negativeelectrode active material, a material which can occlude and release ionsfunctioning as carriers (hereinafter referred to as carrier ions), suchas carbon or silicon, is used. For example, silicon or phosphorus-dopedsilicon can occlude about four times as many carrier ions as carbon andthus has higher theoretical capacity than carbon and is advantageous inincreasing the capacity of the power storage device.

However, when the amount of carrier ions which are occluded isincreased, the volume of an active material greatly changes inaccordance with occlusion and release of carrier ions incharge/discharge cycle, resulting in lower adhesion between a currentcollector and silicon and deterioration in battery characteristics dueto charge/discharge. Accordingly, a layer formed using silicon is formedover a current collector and a layer formed using a graphite is formedover the layer formed using silicon, thereby reducing deterioration inbattery characteristics due to expansion and contraction of the layerformed using silicon (see Patent Document 1).

Silicon has lower electric conductivity than carbon; thus, by coveringsurfaces of silicon particles with a graphite and forming an activematerial layer including the silicon particles over a current collector,a negative electrode in which the resistivity of the active materiallayer is reduced is manufactured.

In recent years, the use of a graphene as a conductive electronicmaterial in semiconductor devices has been studied. A graphene refers toa sheet of carbon molecules with a thickness of one atomic layer havingdouble bonds (also referred to as graphite bonds or sp² bonds).

A graphene is chemically stable and has favorable electriccharacteristics and thus has been expected to be applied to channelregions of transistors, vias, wirings, and the like included in thesemiconductor devices. In addition, particles of an active material arecovered with a graphite or a graphene in order to increase theconductivity of a material for an electrode in a lithium-ion battery(see Patent Document 2).

Further, in a power storage device, a positive electrode and a negativeelectrode are each provided with a plurality of protrusions so as toincrease the capacity; in such a power storage device, a top portion ofeach of the plurality of protrusions of the positive electrode and thenegative electrode is provided with an insulator in order to reducepressure applied to a separator between the electrodes when the volumeof the electrodes increases owing to charge/discharge (see PatentDocuments 3 to 5).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2001-283834-   [Patent Document 2] Japanese Published Patent Application No.    2011-029184-   [Patent Document 3] Japanese Published Patent Application No.    2010-219030-   [Patent Document 4] Japanese Published Patent Application No.    2010-239122-   [Patent Document 5] Japanese Published Patent Application No.    2010-219392

DISCLOSURE OF INVENTION

When a layer formed using silicon provided over a current collector iscovered with a layer formed using a graphite, since the thickness of thelayer formed using a graphite is large, e.g., submicron to micron, theamount of carrier ions transferred between an electrolyte and the layerformed using silicon is reduced. In addition, in an active materiallayer including silicon particles covered with a graphite, the amount ofsilicon contained in the active material layer is reduced. Consequently,the amount of reaction between silicon and carrier ions is reduced,which causes a reduction in charge/discharge capacity and makes itdifficult to perform charge/discharge at high speed in a power storagedevice.

In addition, even when particles of an active material are covered witha graphene, it is difficult to suppress expansion and contraction of thevolume of the particles of the active material owing to repeatingcharge/discharge and to suppress pulverization of the particles of theactive material due to the expansion and the contraction.

In view of the above, an embodiment of the present invention provides apower storage device in which charge capacity and discharge capacity arehigh, charge/discharge can be performed at high speed, and deteriorationin battery characteristics due to charge/discharge is small.

An embodiment of the present invention is a power storage deviceincluding a negative electrode. The negative electrode includes a commonportion, a plurality of protrusions protruding from the common portion,and a graphene provided over the common portion and the plurality ofprotrusions. At least the plurality of protrusions function as an activematerial. Axes of the plurality of protrusions are oriented in the samedirection.

Note that in a lateral cross-sectional shape, an area of a bottomportion of each of the plurality of protrusions which is in contact withthe common portion may be larger than an area of a top portion of eachof the plurality of protrusions. In addition, in a longitudinalcross-sectional shape, a width of a bottom portion of each of theplurality of protrusions which is in contact with the common portion maybe larger than a width of a top portion of each of the plurality ofprotrusions.

An embodiment of the present invention is a power storage deviceincluding a negative electrode. The negative electrode includes acurrent collector and an active material layer provided over the currentcollector. The active material layer includes a plurality of protrusionsprotruding from the current collector, and a graphene provided over theplurality of protrusions. Axes of the plurality of protrusions areoriented in the same direction. Note that a common portion may beprovided between the current collector and the plurality of protrusions.

Note that a cross-sectional area of a bottom portion of each of theplurality of protrusions which is in contact with the current collectormay be larger than a cross-sectional area of a top portion of each ofthe plurality of protrusions. In addition, in a longitudinalcross-sectional shape, a width of a bottom portion of each of theplurality of protrusions which is in contact with the current collectormay be larger than a width of a top portion of each of the plurality ofprotrusions.

An embodiment of the present invention is a power storage deviceincluding a common portion, a plurality of protrusions protruding fromthe common portion, and a graphene provided over the common portion andthe plurality of protrusions. At least the plurality of protrusionsfunction as an active material. The plurality of protrusions havetranslation symmetry when viewed from the above.

Note that in a lateral cross-sectional shape, an area of a bottomportion of each of the plurality of protrusions which is in contact withthe common portion may be larger than an area of a top portion of eachof the plurality of protrusions. In addition, in a longitudinalcross-sectional shape, a width of a bottom portion of each of theplurality of protrusions which is in contact with the common portion maybe larger than a width of a top portion of each of the plurality ofprotrusions.

An embodiment of the present invention is a power storage deviceincluding a current collector and an active material layer provided overthe current collector. The active material layer includes a plurality ofprotrusions protruding from the current collector and a grapheneprovided over the plurality of protrusions. The plurality of protrusionshave translation symmetry when viewed from the above. Note that a commonportion may be provided between the current collector and the pluralityof protrusions.

Note that a cross-sectional area of a bottom portion of each of theplurality of protrusions which is in contact with the current collectormay be larger than a cross-sectional area of a top portion of each ofthe plurality of protrusions. In addition, in a longitudinalcross-sectional shape, a width of a bottom portion of each of theplurality of protrusions which is in contact with the current collectormay be larger than a width of a top portion of each of the plurality ofprotrusions.

In the electrode, the common portion means a region which covers anentire surface of the current collector and is formed using a materialsimilar to that of the plurality of protrusions. Further, an axis ofeach of the plurality of protrusions means a straight line which passesa top portion of the protrusion (or the center of a top surface of theprotrusion) and the center of a bottom surface of the protrusion whichis in contact with the common portion or the current collector. That is,the axis is a straight line which passes the center of the longitudinaldirection of the protrusion. When the axes of the plurality ofprotrusions are oriented in the same direction, the straight lines ofthe plurality of protrusions are substantially parallel with each other.Specifically, the angle between the straight lines of the plurality ofprotrusions is less than or equal to 10 degrees, preferably less than orequal to 5 degrees. As described above, the plurality of protrusions arestructures which are formed by etching and different from whisker-likestructures which extend in various directions.

The common portion and the plurality of protrusions may be formed usingsilicon, silicon to which an impurity imparting a conductivity type suchas phosphorus or boron is added, single crystal silicon, polycrystallinesilicon, or amorphous silicon. Alternatively, the common portion may beformed using single crystal silicon or polycrystalline silicon, and theplurality of protrusions may be formed using amorphous silicon. Furtheralternatively, the common portion and part of the plurality ofprotrusions may be formed using single crystal silicon orpolycrystalline silicon, and the other part of the plurality ofprotrusions may be formed using amorphous silicon.

The plurality of protrusions may each have a columnar shape, a conicalor pyramidal shape, a plate-like shape, or a pipe-like shape.

A protective layer may be provided between a top portion of each of theplurality of protrusions and the graphene.

Surfaces of the plurality of protrusions may be covered with a graphene.A graphene includes a single-layer graphene and a multilayer graphene inits category. A graphene may contain oxygen at a concentration of higherthan or equal to 2 at. % and lower than or equal to 11 at. %, preferablyhigher than or equal to 3 at. % and lower than or equal to 10 at. %.

In addition, an insulating layer functioning as a spacer may be providedover the graphene of the negative electrode.

The insulating layer provided over the graphene of the negativeelectrode has a top surface having a dot-like shape, a rectangularshape, a lattice-like shape, or the like and is provided over a topportion of at least one protrusion. The insulating layer provided overthe graphene of the negative electrode is preferably formed using anorganic material such as an acrylic resin, an epoxy resin, a siliconeresin, polyimide, or polyamide, or low-melting-point glass such as glasspaste, glass frit, or glass ribbon. The insulating layer provided overthe graphene of the negative electrode may contain electrolyte solute.The thickness of the insulating layer provided over the graphene of thenegative electrode is preferably greater than or equal to 1 μm and lessthan or equal to 10 μm, more preferably greater than or equal to 2 μmand less than or equal to 7 μm.

An embodiment of the present invention is a method for manufacturing anelectrode, including the steps of forming a mask over a siliconsubstrate, etching part of the silicon substrate to form a commonportion and a plurality of protrusions protruding from the commonportion, and forming a graphene over the common portion and theplurality of protrusions.

An embodiment of the present invention is a method for manufacturing anelectrode, including the steps of forming a silicon layer over a currentcollector, forming a mask over the silicon layer, etching part of thesilicon layer to form a plurality of protrusions protruding from thecurrent collector, and forming a graphene at least over the plurality ofprotrusions.

An embodiment of the present invention is a method for manufacturing anegative electrode, including the steps of forming a mask over a siliconsubstrate, etching part of the silicon substrate to form a commonportion and a plurality of protrusions protruding from the commonportion, forming another mask by reducing the mask in size by oxygenplasma treatment or the like, etching at least part of the plurality ofprotrusions protruding from the common portion to form a common portionand a plurality of protrusions so that in a lateral cross-sectionalshape, an area of a bottom portion of each of the plurality ofprotrusions which is in contact with the common portion is larger thanan area of a top portion of each of the plurality of protrusions, andforming a graphene over the common portion and the plurality ofprotrusions.

An embodiment of the present invention is a method for manufacturing anegative electrode, including the steps of forming a silicon layer overa current collector, forming a mask over the silicon layer, etching partof the silicon layer to form a plurality of protrusions protruding fromthe current collector, forming another mask by reducing the mask in sizeby resist slimming or the like, etching at least part of the pluralityof protrusions protruding from the current collector to form a pluralityof protrusions so that in a lateral cross-sectional shape, an area of abottom portion of each of the plurality of protrusions which is incontact with the current collector is larger than an area of a topportion of each of the plurality of protrusions, and forming a grapheneover the current collector and the plurality of protrusions.

An embodiment of the present invention is a method for manufacturing anegative electrode, including the steps of forming a mask over a siliconsubstrate, etching part of the silicon substrate to form a commonportion and a plurality of protrusions protruding from the commonportion, forming a graphene over the common portion and the plurality ofprotrusions, and forming an insulating layer functioning as a spacerover the graphene.

An embodiment of the present invention is a method for manufacturing anegative electrode, including the steps of forming a silicon layer overa current collector, forming a mask over the silicon layer, etching partof the silicon layer to form a plurality of protrusions protruding fromthe current collector, forming a graphene at least over the plurality ofprotrusions, and forming an insulating layer functioning as a spacerover the graphene.

An active material of an electrode includes a common portion and aplurality of protrusions which protrude from the common portion. Axes ofthe plurality of protrusions are oriented in the same direction and theprotrusions protrude in the direction perpendicular to the commonportion, so that the density of the protrusions in the electrode can beincreased and the surface area of the active material can be increased.A space is provided between the plurality of protrusions. Further, agraphene is provided over the active material. Thus, even when theactive material expands in charging, contact between the protrusions canbe reduced. Even when the active material is separated, the activematerial can be prevented from being broken. The plurality ofprotrusions have translation symmetry and formed with high uniformity inthe negative electrode, so that local reaction can be reduced in each ofthe positive electrode and the negative electrode, and carrier ions andthe active material react with each other uniformly between the positiveelectrode and the negative electrode. Consequently, in the case wherethe negative electrode is used for a power storage device, high-speedcharge/discharge becomes possible, and breakdown and separation of theactive material due to charge/discharge can be suppressed, that is, apower storage device with improved charge/discharge cyclecharacteristics can be manufactured.

In accordance with an embodiment of the present invention, in a lateralcross-sectional shape, an area of a bottom portion of each of theplurality of protrusions which is in contact with the current collectoror the common portion is larger than an area of a top portion of each ofthe plurality of protrusions. That is, each of the plurality ofprotrusions has a shape in which the bottom portion is wider than thetop portion. Thus, the mechanical strength is improved, anddeterioration such as pulverization or separation due to expansion andcontraction of the active material caused by charge/discharge reactioncan be controlled. Further, assembly of the battery is done with the useof the plurality of protrusions each having a shape in which the bottomportion is wider than the top portion for the negative electrode; inthat case, even when the top portions of the plurality of protrusionsare broken by being in contact with the separator or the like, thebottom portions of the plurality of protrusions having high strengthtend to remain. Accordingly, the yield of the assembly to manufacturethe battery can be improved.

When the surface of the active material is in contact with anelectrolyte in the power storage device, the electrolyte and the activematerial react with each other, so that a film is formed over a surfaceof the active material. The film is called a solid electrolyte interface(SEI) and considered necessary to relieve the reaction between theactive material and the electrolyte and for stabilization. However, whenthe thickness of the film is increased, carrier ions are less likely tobe occluded by the active material, leading to a problem such as areduction in conductivity of carrier ions between the active materialand the electrolyte. A graphene covering the active material cansuppress an increase in thickness of the film, so that a decrease inconductivity of carrier ions can be suppressed.

Silicon has lower electric conductivity than carbon, and the electricconductivity is further reduced when silicon becomes amorphous due tocharge/discharge. Thus, a negative electrode in which silicon is used asan active material has high resistivity. However, since a graphene hashigh conductivity, by covering silicon with a graphene, electrons cantransfer at sufficiently high speed in a graphene through which carrierions pass. In addition, a graphene has a thin sheet-like shape; byproviding a graphene over a plurality of protrusions, the amount ofsilicon in the active material layer can be increased and carrier ionscan transfer more easily than in a graphite. As a result, theconductivity of carrier ions can be increased, reaction between siliconthat is an active material and carrier ions can be increased, andcarrier ions can be easily occluded by the active material. Accordingly,a power storage device including the negative electrode can performcharge/discharge at high speed.

When an insulating layer functioning as a spacer is provided over agraphene of a negative electrode, it is not necessary to provide aseparator between the negative electrode and a positive electrode, sothat the distance between the negative electrode and the positiveelectrode can be shortened. As a result, the amount of carrier ionstransferring between the positive electrode and the negative electrodecan be increased.

In accordance with an embodiment of the present invention, at least anactive material including a plurality of protrusions and a grapheneprovided over the active material are provided, whereby a power storagedevice which has high charge/discharge capacity and less deteriorationdue to charge/discharge and can perform charge/discharge at high speedcan be provided.

In accordance with an embodiment of the present invention, an activematerial including a plurality of protrusions and a graphene providedover the active material are provided, and in a lateral cross-sectionalshape, an area of a bottom portion of each of the plurality ofprotrusions which is in contact with a current collector or a commonportion is larger than an area of a top portion of each of the pluralityof protrusions, whereby a power storage device which has highcharge/discharge capacity and less deterioration due to charge/dischargeand can perform charge/discharge at high speed can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C each illustrate a negative electrode.

FIGS. 2A to 2D each illustrate a shape of a protrusion included in anegative electrode.

FIGS. 3A to 3D each illustrate a negative electrode.

FIGS. 4A to 4C illustrate a method for manufacturing a negativeelectrode.

FIGS. 5A to 5D each illustrate a negative electrode.

FIGS. 6A to 6C illustrate a method for manufacturing a negativeelectrode.

FIGS. 7A to 7C each illustrate a negative electrode.

FIGS. 8A to 8D each illustrate a shape of a protrusion included in anegative electrode.

FIGS. 9A to 9D each illustrate a negative electrode.

FIGS. 10A to 10C illustrate a method for manufacturing a negativeelectrode.

FIGS. 11A and 11B illustrate a method for manufacturing a negativeelectrode.

FIGS. 12A to 12D each illustrate a negative electrode.

FIGS. 13A to 13C illustrate a method for manufacturing a negativeelectrode.

FIGS. 14A and 14B illustrate a method for manufacturing a negativeelectrode.

FIGS. 15A to 15C each illustrate a negative electrode.

FIGS. 16A to 16C each illustrate a shape of a spacer.

FIGS. 17A to 17C illustrate a method for manufacturing a negativeelectrode.

FIGS. 18A to 18D illustrate a method for manufacturing a negativeelectrode.

FIGS. 19A to 19C illustrate a method for manufacturing a negativeelectrode.

FIGS. 20A to 20C illustrate a positive electrode.

FIGS. 21A and 21B illustrate a positive electrode.

FIG. 22 illustrates a power storage device.

FIG. 23 illustrates electronic devices.

FIGS. 24A to 24C illustrate an electronic device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments will be described with reference to thedrawings. However, the embodiments can be implemented in many differentmodes, and it will be readily appreciated by those skilled in the artthat modes and details thereof can be changed in various ways withoutdeparting from the spirit and the scope of the present invention. Thus,the present invention should not be interpreted as being limited to thedescription below of the embodiments.

Embodiment 1

In this embodiment, a structure of a negative electrode of a powerstorage device which is less deteriorated through charge/discharge andhas excellent charge/discharge cycle characteristics and a manufacturingmethod thereof will be described with reference to FIGS. 1A to 1C, FIGS.2A to 2D, FIGS. 3A to 3D, FIGS. 4A to 4C, FIGS. 5A to 5D, and FIGS. 6Ato 6C.

FIG. 1A is a cross-sectional view of a negative electrode 206. Thenegative electrode 206 functions as an active material.

Note that an active material refers to a material that relates toocclusion and release of carrier ions. An active material layercontains, in addition to the active material, one or more of aconductive additive, a binder, a graphene, and the like. Thus, theactive material and the active material layer are distinguished fromeach other.

A secondary battery in which lithium ions are used as carrier ions isreferred to as a lithium-ion secondary battery. As examples of carrierions which can be used instead of lithium ions, alkali-metal ions suchas sodium ions and potassium ions; alkaline-earth metal ions such ascalcium ions, strontium ions, and barium ions; beryllium ions; magnesiumions; and the like are given.

A specific structure of the negative electrode 206 will be describedwith reference to FIGS. 1B and 1C. Typical examples of the negativeelectrode 206 are a negative electrode 206 a and a negative electrode206 b in FIGS. 1B and 1C, respectively.

FIG. 1B is an enlarged cross-sectional view of the negative electrode206 a. The negative electrode 206 a includes an active material 202 anda graphene 204 provided over the active material 202. The activematerial 202 includes a common portion 202 a and a plurality ofprotrusions 202 b which protrude from the common portion 202 a. Thegraphene 204 covers at least part of the active material 202.Alternatively, the graphene 204 may cover surfaces of the common portion202 a and the plurality of protrusions 202 b of the active material 202.

As the active material 202, any one of silicon, germanium, tin,aluminum, and the like, which can occlude and release ions serving ascarriers, is used. Silicon which has high theoretical charge/dischargecapacity is preferably used as the active material 202. Alternatively,silicon to which an impurity element imparting one conductivity type,such as phosphorus or boron, is added may be used. Silicon to which theimpurity element imparting one conductivity type, such as phosphorus orboron, is added has higher conductivity, so that the conductivity of thenegative electrode can be increased. Accordingly, discharge capacity canbe further improved as compared to a power storage device including anegative electrode in which silicon is used as the active material 202.

The common portion 202 a serves as a base layer of the plurality ofprotrusions 202 b. The common portion 202 a is a continuous layer and iscontact with the plurality of protrusions 202 b.

Each of the protrusions 202 b can have any of the following shapes asappropriate: a columnar shape such as a cylindrical shape 221 (see FIG.2A) or a prismatic shape, a conical shape 222 (see FIG. 2B) or apyramidal shape, a plate-like shape 223 (see FIG. 2C), a pipe-like shape224 (see FIG. 2D), and the like. Note that the top or the edge of theprotrusion 202 b may be curved. In FIG. 1B, a cylindrical protrusion isused as the protrusion 202 b.

A top view of the electrode in this embodiment will be described withreference to FIGS. 3A to 3D.

FIG. 3A is a top view illustrating the common portion 202 a and theplurality of protrusions 202 b which protrude from the common portion202 a. The plurality of protrusions 202 b which have circular top shapesare arranged. FIG. 3B is a top view after movement of the plurality ofprotrusions 202 b in FIG. 3A in the direction a. In FIGS. 3A and 3B, theplurality of protrusions 202 b are provided at the same positions. Thatis, the plurality of protrusions 202 b illustrated in FIG. 3A havetranslation symmetry. Here, the plurality of protrusions 202 b in FIG.3A move in the direction a; however, the same result as FIG. 3B can beobtained after movement in the direction b or c.

The proportion of the protrusion 202 b in the unit of symmetry which isdenoted by a dashed line 205 is preferably higher than or equal to 25%and lower than or equal to 60%. That is, the proportion of a space (aregion without the protrusion) in the unit of symmetry is preferablyhigher than or equal to 40% and lower than or equal to 75%. When theproportion of the protrusion 202 b in the unit of symmetry is higherthan or equal to 25%, the theoretical charge/discharge capacity of thenegative electrode can be higher than or equal to about 1000 mAh/g. Inaddition, by setting the proportion of the protrusion 202 b in the unitof symmetry to lower than or equal to 60%, also when thecharge/discharge capacity is maximum (i.e., theoretical capacity) andthe protrusions expand, the adjacent protrusions are not in contact witheach other and can be prevented from being broken. As a result, highcharge/discharge capacity can be achieved and deterioration of thenegative electrode due to charge/discharge can be reduced.

FIG. 3C is a top view illustrating the common portion 202 a and aplurality of protrusions which protrude from the common portion 202 a.The protrusion 202 b which has a circular top shape and a protrusion 202c which has a square top shape are alternately arranged. FIG. 3D is atop view after movement of the plurality of protrusions 202 b and 202 cin the direction b. In the top views of FIGS. 3C and 3D, the pluralityof protrusions 202 b and 202 c are provided at the same positions. Thatis, the plurality of protrusions 202 b and 202 c illustrated in FIG. 3Chave translation symmetry.

By providing the plurality of protrusions such that they havetranslation symmetry, variation in electron conductivity among theplurality of protrusions can be reduced. Accordingly, local reaction inthe positive electrode and the negative electrode can be reduced,reaction between carrier ions and the active material can occuruniformly, and diffusion overvoltage (concentration overvoltage) can beprevented, so that the reliability of battery characteristics can beincreased.

The common portion 202 a and the plurality of protrusions 202 b can havea single crystal structure or a polycrystalline structure asappropriate. Alternatively, the common portion 202 a can have a singlecrystal structure or a polycrystalline structure, and the plurality ofprotrusions 202 b can have an amorphous structure. Furtheralternatively, the common portion 202 a and part of the plurality ofprotrusions 202 b can have a single crystal structure or apolycrystalline structure, and the other part of the plurality ofprotrusions 202 b can have an amorphous structure. Note that the part ofthe plurality of protrusions 202 b includes at least a region in contactwith the common portion 202 a.

The interface between the common portion 202 a and the plurality ofprotrusions 202 b is not clear. Accordingly, in the active material 202,a plane including the deepest depression among depressions between theplurality of protrusions 202 b and parallel with a plane where theprotrusions 202 b are formed is defined as an interface 233 between thecommon portion 202 a and the plurality of protrusions 202 b.

In addition, the longitudinal directions of the plurality of protrusions202 b are oriented in the same direction. That is, axes 231 of theplurality of protrusions 202 b are parallel with each other. Further,preferably, the plurality of protrusions 202 b have substantially thesame shapes. With such a structure, the volume of the active materialcan be controlled. Further, the axis 231 of the protrusion is a straightline which passes the top of the protrusion (or the center of a topsurface of the protrusion) and the center of a bottom surface of theprotrusion which is in contact with the common portion. That is, theaxis is a straight line which passes the center of the longitudinaldirection of the protrusion. When the axes of the plurality ofprotrusions are oriented in the same direction, the axes of theplurality of protrusions are substantially parallel with each other.Specifically, the angle between the axes of the plurality of protrusionsis less than or equal to 10 degrees, preferably less than or equal to 5degrees.

The direction in which the plurality of protrusions 202 b extend fromthe common portion 202 a is referred to as a longitudinal direction, anda cross-sectional shape parallel with the longitudinal direction isreferred to as a longitudinal cross-sectional shape. In addition, across-sectional shape of a plane substantially perpendicular to thelongitudinal direction of the plurality of protrusions 202 b is referredto as a lateral cross-sectional shape.

The width of the protrusion 202 b in a lateral cross-sectional shape isgreater than or equal to 0.1 μm and less than or equal to 1 μm,preferably greater than or equal to 0.2 μm and less than or equal to 0.5μm. The height of the protrusion 202 b is five times to hundred times,preferably ten times to fifty times, of the width of the protrusion,typically, greater than or equal to 0.5 μm and less than or equal to 100μm, preferably greater than or equal to 1 μm and less than or equal to50 μm.

With the width of the protrusion 202 b in a lateral cross-sectionalshape being greater than or equal to 0.1 μm, the charge/dischargecapacity can be increased. With the width of the protrusion 202 b in alateral cross-sectional shape being less than or equal to 1 μm, evenwhen the plurality of protrusions expand or contract in charge anddischarge, the protrusions can be prevented from being broken. Inaddition, with the height of the protrusion 202 b being greater than orequal to 0.5 μm, the charge/discharge capacity can be increased. Withthe height of the protrusion 202 b being less than or equal to 100 μm,even when the plurality of protrusions expand or contract in charge anddischarge, the protrusions can be prevented from being broken.

The height of the protrusion 202 b is a distance between the commonportion 202 a and the top (or the center of the top surface) of theprotrusion 202 b in the direction parallel with the axis which passesthe top, in a longitudinal cross-sectional shape.

The plurality of protrusions 202 b are provided over the common portion202 a with a predetermined distance therebetween. The distance betweenthe plurality of protrusions 202 b is preferably 1.29 times to 2 timesof the width of the protrusion 202 b. Consequently, even when the volumeof the protrusion 202 b increases due to charge of the power storagedevice including the negative electrode, the protrusions 202 b are notin contact with each other and can be prevented from being broken, andmoreover, a reduction in charge/discharge capacity of the power storagedevice can be prevented.

The graphene 204 functions as a conductive additive. In addition, thegraphene 204 functions as an active material in some cases.

The graphene 204 includes a single-layer graphene and a multilayergraphene in its category. The graphene 204 has a sheet-like shape with alength of several micrometers.

The single-layer graphene refers to a sheet of carbon molecules havingsp² bonds with a thickness of one atomic layer and is very thin. Inaddition, six-membered rings each composed of carbon atoms are connectedin the planar direction, and poly-membered rings each formed when acarbon-carbon bond in part of a six-membered ring is broken, such as aseven-membered ring, an eight-membered ring, a nine-membered ring, and aten-membered ring, are partly formed.

A poly-membered ring is composed of a carbon atom and an oxygen atom insome cases. Further, an oxygen atom is bonded to one of carbon atoms ina poly-membered ring composed of the carbon atoms in some cases. Theabove poly-membered ring is formed when a carbon-carbon bond in part ofa six-membered ring is broken and an oxygen atom is bonded to a carbonatom whose bond is broken. Accordingly, an opening functioning as a paththrough which ions can transfer is included in the bond between thecarbon atom and the oxygen atom. That is, as the proportion of oxygenatoms included in a graphene is higher, the proportion of openings eachfunctioning as a path through which ions can transfer is increased.

When the graphene 204 contains oxygen, the proportion of oxygen in theconstituent atoms of the graphene is higher than or equal to 2 at. % andlower than or equal to 11 at. %, preferably higher than or equal to 3at. % and lower than or equal to 10 at. %. As the proportion of oxygenis lower, the conductivity of the graphene can be increased. As theproportion of oxygen is higher, more openings serving as paths of ionsin the graphene can be formed.

When the graphene 204 is a multilayer graphene, the graphene 204includes a plurality of single-layer graphenes, typically, two tohundred single-layer graphenes and thus is very thin. Since thesingle-layer graphene contains oxygen, the interlayer distance betweenthe graphenes is greater than 0.34 nm and less than or equal to 0.5 nm,preferably greater than or equal to 0.38 nm and less than or equal to0.42 nm, more preferably greater than or equal to 0.39 nm and less thanor equal to 0.41 nm. In a general graphite, the interlayer distancebetween the single-layer graphenes is 0.34 nm. Since the interlayerdistance in the graphene 204 is longer than that in a general graphite,ions can easily transfer in a direction parallel with a surface of thesingle-layer graphene. In addition, the graphene 204 contains oxygen andincludes a single-layer graphene or a multilayer graphene in which apoly-membered ring is formed and thus includes openings in places. Thus,in the case where the graphene 204 is a multilayer graphene, ions cantransfer in the direction parallel with a surface of the single-layergraphene, i.e., through a gap between the single-layer graphenes, and inthe direction perpendicular to a surface of the graphene, i.e., throughan opening formed in each of the single-layer graphenes.

With the use of silicon as a negative electrode active material, thetheoretical capacity is higher than in the case where a graphite is usedas the active material; thus, silicon is advantageous in downsizing thepower storage device.

In addition, since the plurality of protrusions 202 b protrude from thecommon portion 202 a in the active material 202 of the negativeelectrode 206, the active material 202 has a larger surface area than aplate-like active material. Axes of the plurality of protrusions areoriented in the same direction and the protrusions protrude in thedirection perpendicular to the common portion, so that the density ofthe protrusions in the negative electrode can be increased and thesurface area of the active material can be further increased. A space isprovided between the plurality of protrusions. Further, a graphene isprovided over the active material. Thus, even when the active materialexpands in charging, contact between the protrusions can be reduced.Further, even when the active material is separated, the graphene canprevent the active material from being broken. The plurality ofprotrusions have translation symmetry and formed with high uniformity inthe negative electrode, so that local reaction can be reduced in each ofthe positive electrode and the negative electrode, and carrier ions andthe active material can react with each other uniformly between thepositive electrode and the negative electrode. Consequently, in the casewhere the negative electrode 206 is used for the power storage device,high-speed charge/discharge becomes possible, and breakdown andseparation of the active material due to charge/discharge can besuppressed, whereby a power storage device with improved cyclecharacteristics can be manufactured. Furthermore, when the shapes of theprotrusions are substantially the same, local charge/discharge can bereduced, and the weight of the active material can be controlled. Inaddition, when the heights of the protrusions are substantially thesame, load can be prevented from being applied locally in themanufacturing process of the battery, which can increase the yield.Accordingly, specifications of the battery can be well controlled.

When the surface of the active material 202 is in contact with anelectrolyte in the power storage device, the electrolyte and the activematerial react with each other, so that a film is formed over a surfaceof the active material. The film is called a solid electrolyte interface(SEI) which is considered necessary for relieving reaction between theactive material and the electrolyte and for stabilization. However, whenthe film is thick, carrier ions are occluded by the active material withdifficulty, which might reduce the conductivity of carrier ions betweenthe active material and the electrolyte.

The graphene 204 covering the active material 202 can suppress anincrease in thickness of the film, so that a decrease in conductivity ofcarrier ions can be suppressed.

A graphene has high conductivity; by covering silicon with a graphene,electrons can transfer at high speed in a graphene. In addition, agraphene has a thin sheet-like shape; by providing a graphene over aplurality of protrusions, the amount of an active material in an activematerial layer can be increased and carrier ions can transfer moreeasily than in a graphite. As a result, the conductivity of carrier ionscan be increased, reaction between silicon that is an active materialand carrier ions can be increased, and carrier ions can be easilyoccluded by the active material. Accordingly, a power storage deviceincluding the above negative electrode can perform charge/discharge athigh speed.

Note that a silicon oxide layer may be provided between the activematerial 202 and the graphene 204. By providing the silicon oxide layerover the active material 202, ions which are carriers are inserted intosilicon oxide in charging of the power storage device. As a result, asilicate compound, e.g., alkali metal silicate such as Li₄SiO₄, Na₄SiO₄,or K₄SiO₄, alkaline earth metal silicate such as Ca₂SiO₄, Sr₂SiO₄, orBa₂SiO₄, Be₂SiO₄, Mg₂SiO₄, or the like is formed. Such a silicatecompound can serve as a path through which carrier ions transfer. Byproviding the silicon oxide layer, expansion of the active material 202can be suppressed. Accordingly, breakdown of the active material 202 canbe suppressed while the charge/discharge capacity is maintained. Indischarging after charging, not all metal ions serving as carrier ionsare released from the silicate compound formed in the silicon oxidelayer and part of the metal ions remain, so that the silicon oxide layeris a mixture layer of silicon oxide and the silicate compound.

In addition, the thickness of the silicon oxide layer is preferablygreater than or equal to 2 nm and less than or equal to 10 nm. With thethickness of the silicon oxide layer being greater than or equal to 2nm, expansion of the active material 202 due to charge/discharge can berelieved. In addition, with the thickness of the silicon oxide layerbeing less than or equal to 10 nm, carrier ions can transfer easily,which can prevent a reduction in charge capacity. By providing thesilicon oxide layer over the active material 202, expansion andcontraction of the active material 202 in charge/discharge can berelieved, so that the active material 202 can be prevented from beingbroken.

Like the negative electrode 206 b illustrated in FIG. 1C, a protectivelayer 207 may be provided between the top of the protrusion 202 b in theactive material 202 and the graphene 204. In that case, a side surfaceof the protrusion 202 b is in contact with the graphene 204.

A conductive layer, a semiconductor layer, or an insulating layer can beused for the protective layer 207 as appropriate. The thickness of theprotective layer 207 is preferably greater than or equal to 100 nm andless than or equal to 10 μm. When the protective layer 207 is formedusing a material whose etching rate is lower than that of the materialfor the active material 202, the protective layer 207 serves as a hardmask when the plurality of protrusions are formed by etching, so thatvariation in height between the plurality of protrusions can be reduced.

Next, a method for manufacturing the negative electrode 206 will bedescribed with reference to FIGS. 4A to 4C. Here, as one mode of thenegative electrode 206, the negative electrode 206 a illustrated in FIG.1B will be described.

As illustrated in FIG. 4A, masks 208 a to 208 e are formed over asilicon substrate 200.

A single crystal silicon substrate or a polycrystalline siliconsubstrate is used as the silicon substrate 200. By using, as the siliconsubstrate, an n-type silicon substrate doped with phosphorus or a p-typesilicon substrate doped with boron, an active material can be used asthe negative electrode without providing the current collector.

The masks 208 a to 208 e can be formed by a photolithography step.Alternatively, the masks 208 a to 208 e can be formed by an inkjetmethod, a printing method, or the like.

The silicon substrate 200 is selectively etched with the use of themasks 208 a to 208 e, so that the active material 202 is formed asillustrated in FIG. 4B. As a method for etching the silicon substrate, adry etching method or a wet etching method can be used as appropriate.Note that when a Bosch process which is a deep etching method is used, ahigh protrusion can be formed.

For example, an n-type silicon substrate is etched with an inductivelycoupled plasma (ICP) apparatus by using, as an etching gas, chlorine,hydrogen bromide, and oxygen, whereby the active material 202 can beformed. The etching time is adjusted such that the common portion 202 aremains. The flow ratio of the etching gas may be adjusted appropriate.For example, the flow ratio of chlorine, hydrogen bromide, and oxygencan be 10:15:3.

As described in this embodiment, the silicon substrate is etched withthe use of the masks, whereby the plurality of protrusions whose axesare oriented in the same direction can be formed. Further, the pluralityof protrusions whose shapes are substantially the same can be formed.

Next, the graphene 204 is formed over the active material 202, so thatthe negative electrode 206 a can be formed as illustrated in FIG. 4C.

As a method for forming the graphene 204, there are a gas phase methodand a liquid phase method. In the gas phase method, after forming, as anucleus, nickel, iron, gold, copper, or an alloy containing such a metalover the active material 202, a graphene is grown from the nucleus in anatmosphere containing hydrocarbon such as methane or acetylene. In theliquid phase method, graphene oxide is provided over the surface of theactive material 202 using a dispersion liquid containing graphene oxide,and then, graphene oxide is reduced to form a graphene.

The dispersion liquid containing graphene oxide is obtained by a methodin which graphene oxide is dispersed in a solvent, a method in whichafter a graphite is oxidized in a solvent, graphite oxide is separatedinto graphene oxide to form a dispersion liquid containing grapheneoxide, and the like. In this embodiment, the graphene 204 is formed overthe active material 202 by using the dispersion liquid containinggraphene oxide which is formed by, after oxidizing graphite, separatinggraphite oxide into graphene oxide.

In this embodiment, graphene oxide is formed by an oxidation methodcalled a Hummers method. A Hummers method is as follows: a sulfuric acidsolution of potassium permanganate or the like is mixed into graphitepowder to cause oxidation reaction; thus, a mixed solution containinggraphite oxide is formed. Graphite oxide contains a functional groupsuch as an epoxy group, a carbonyl group including a carboxyl group, ora hydroxyl group due to oxidation of carbon in graphite. Accordingly,the interlayer distance between adjacent graphenes of a plurality ofgraphenes in graphite oxide is longer than the interlayer distance ofgraphite. Then, ultrasonic vibration is transferred to the mixedsolution containing graphite oxide, so that the graphite oxide whoseinterlayer distance is long can be cleaved to separate graphene oxideand to form a dispersion liquid containing graphene oxide. Note that amethod for forming graphene oxide other than a Hummers method can beused as appropriate.

Graphene oxide includes an epoxy group, a carbonyl group including acarboxyl group, a hydroxyl group, or the like. Such a substituent hashigh polarity, so that graphene oxides are likely to disperse in aliquid having a polarity. In particular, since hydrogen is ionized in aliquid having a polarity, graphene oxide including a carbonyl group isionized and different graphene oxides are more likely to disperse.Accordingly, in a liquid having a polarity, graphene oxides disperseuniformly, and in a later step, graphene oxides can be provideduniformly over the surface of the active material 202.

As a method of soaking the active material 202 in the dispersion liquidcontaining graphene oxide to provide graphene oxide over the activematerial 202, a coating method, a spin coating method, a dipping method,a spray method, an electrophoresis method, or the like may be employed.Alternatively, these methods may be combined as appropriate. With theuse of an electrophoresis method, ionized graphene oxide can beelectrically transferred to the active material, whereby graphene oxidecan be provided also on a surface of the common portion which is not incontact with the plurality of protrusions. Accordingly, even when theplurality of protrusions are high, graphene oxide can be provideduniformly over the surfaces of the common portion and the plurality ofprotrusions.

In a method for reducing graphene oxide provided over the activematerial 202, heating may be performed at higher than or equal to 150°C., preferably higher than or equal to 200° C. and lower than or equalto the temperature which the active material 202 can withstand, in avacuum, air, an atmosphere of an inert gas (nitrogen, a rare gas, or thelike), or the like. By being heated at a higher temperature and for alonger time, graphene oxide is reduced to a higher extent so that agraphene with high purity (i.e., with a low concentration of elementsother than carbon) can be obtained. In addition, there is also a methodin which graphene oxide is soaked in a reducing solution to be reduced.

Since a graphite is treated with sulfuric acid according to a Hummersmethod, a sulfone group and the like are also bonded to graphene oxide,and its decomposition (release) is caused at higher than or equal to200° C. and lower than or equal to 300° C., preferably higher than orequal to 200° C. and lower than or equal to 250° C. Thus, in a methodfor reducing graphene oxide by heating, graphene oxide is preferablyreduced at higher than or equal to 200° C.

Through the reduction treatment, adjacent graphenes are bonded to eachother to form a huge net-like or sheet-like shape. Further, through thereduction treatment, openings are formed in the graphenes due to therelease of oxygen. Furthermore, the graphenes overlap with each other inparallel with a surface of a substrate. As a result, graphenes in whichions can transfer between layers and in openings is formed.

In accordance with this embodiment, the negative electrode 206 aillustrated in FIG. 1B is formed.

A protective layer is formed over the silicon substrate 200, the masks208 a to 208 e are formed over the protective layer, and separatedprotective layers 207 are formed with the use of the masks 208 a to 208e (see FIG. 1C). After that, with the use of the masks 208 a to 208 eand the separated protective layers, the silicon substrate 200 isselectively etched, whereby the negative electrode 206 b illustrated inFIG. 1C can be formed. When the plurality of protrusions 202 b are high,that is, the etching time is long, the masks are thinned gradually inthe etching step and part of the masks are removed to expose the siliconsubstrate 200. Accordingly, there is variation in height between theprotrusions. However, by using the separated protective layers 207 ashard masks, the silicon substrate 200 can be prevented from beingexposed, so that variation in height between the protrusions can bereduced.

Embodiment 2

In this embodiment, a negative electrode having a structure differentfrom that of Embodiment 1 and a method for manufacturing the negativeelectrode will be described with reference to FIGS. 5A to 5D and FIGS.6A to 6C. The negative electrode described in this embodiment isdifferent from that of Embodiment 1 in that a current collector isprovided.

FIG. 5A is a cross-sectional view of a negative electrode 216. In thenegative electrode 216, an active material layer 215 is provided over acurrent collector 211.

A specific structure of the negative electrode 216 will be describedwith reference to FIGS. 5B to 5D. Typical examples of the activematerial layer 215 included in the negative electrode 216 are an activematerial layer 215 a, an active material layer 215 b, and an activematerial layer 215 c in FIGS. 5B, 5C, and 5D, respectively.

FIG. 5B is an enlarged cross-sectional view of the current collector 211and the active material layer 215 a. The active material layer 215 a isprovided over the current collector 211. The active material layer 215 aincludes an active material 212 and a graphene 214 provided over theactive material 212. The active material 212 includes a common portion212 a and a plurality of protrusions 212 b which protrude from thecommon portion 212 a. In addition, the longitudinal directions of theplurality of protrusions 212 b are oriented in the same direction. Thatis, axes 241 of the plurality of protrusions 212 b are oriented in thesame direction. Further, the axis 241 of the protrusion is a straightline which passes the top of the protrusion (or the center of a topsurface of the protrusion) and the center of a bottom surface of theprotrusion which is in contact with the common portion. That is, theaxis is a straight line which passes the center of the longitudinaldirection of the protrusion.

When the current collector 211 is formed using a metal material thatforms silicide, in the current collector 211, a silicide layer may beformed on the side in contact with the active material 212. In the casewhere a metal material that forms silicide is used to form the currentcollector 211, titanium silicide, zirconium silicide, hafnium silicide,vanadium silicide, niobium silicide, tantalum silicide, chromiumsilicide, molybdenum silicide, cobalt silicide, nickel silicide, or thelike is formed as a silicide layer.

The current collector 211 can be formed using a highly conductivematerial such as a metal typified by stainless steel, gold, platinum,zinc, iron, aluminum, copper, or titanium, or an alloy thereof. Notethat the current collector 211 is preferably formed using an aluminumalloy to which an element which improves heat resistance, such assilicon, titanium, neodymium, scandium, or molybdenum, is added.Alternatively, the current collector 211 may be formed using a metalelement which forms silicide by reacting with silicon. Examples of themetal element which forms silicide by reacting with silicon includezirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium,molybdenum, tungsten, cobalt, nickel, and the like.

The current collector 211 can have a foil-like shape, a plate-like shape(a sheet-like shape), a net-like shape, a punching-metal shape, anexpanded-metal shape, or the like as appropriate.

The active material 212 can be formed using a material similar to thatof the active material 202 in Embodiment 1 as appropriate.

The common portion 212 a is a layer which serves as a base layer of theplurality of protrusions 212 b and is continuous over the currentcollector 211, similarly to the common portion 202 a in Embodiment 1. Inaddition, the common portion 212 a and the plurality of protrusions 212b are in contact with each other.

The plurality of protrusions 212 b can have the same shape as theplurality of protrusions 202 b in Embodiment 1 as appropriate.

The common portion 212 a and the plurality of protrusions 212 b can havea single crystal structure, a polycrystalline structure, or an amorphousstructure as appropriate. In addition, the common portion 212 a and theplurality of protrusions 212 b can have a crystalline structure which isintermediate of these structures, such as a microcrystalline structure.Alternatively, the common portion 212 a can have a single crystalstructure or a polycrystalline structure, and the plurality ofprotrusions 212 b can have an amorphous structure. Furtheralternatively, the common portion 212 a and part of the plurality ofprotrusions 212 b can have a single crystal structure or apolycrystalline structure, and the other part of the plurality ofprotrusions 212 b can have an amorphous structure. Note that the part ofthe plurality of protrusions 212 b includes at least a region in contactwith the common portion 212 a.

The width or height of the protrusion 212 b can be the same as theprotrusion 202 b in Embodiment 1.

As the graphene 214, the graphene 204 in Embodiment 1 can be used asappropriate.

Like the active material layer 215 b in FIG. 5C, the negative electrode216 may have a structure in which the common portion is not provided,the plurality of protrusions 212 b which are separated from each otherare provided over the current collector 211, and the graphene 214 isformed over the current collector 211 and the plurality of protrusions212 b. Axes 251 of the plurality of protrusions 212 b are oriented inthe same direction. The axis 251 of the protrusion 212 b is a straightline which passes the top of the protrusion (or the center of a topsurface of the protrusion) and the center of a bottom surface of theprotrusion 212 b which is in contact with the current collector 211.That is, the axis is a straight line which passes the center of thelongitudinal direction of the protrusion.

The graphene 214 is in contact with part of the current collector 211,so that electrons can flow easily in the graphene 214 and reactionbetween the carrier ions and the active material can be improved.

Like the active material layer 215 c illustrated in FIG. 5D, aprotective layer 217 may be provided between the top of the protrusion212 b and the graphene 214. A material similar to that for theprotective layer 207 described in Embodiment 1 can be used for theprotective layer 217 as appropriate. Description is given using theactive material 212 in FIG. 5B here, but the protective layer 217 may beprovided over the active material in FIG. 5C.

In the negative electrode described in this embodiment, the activematerial layer can be provided using the current collector 211 as asupport. Accordingly, when the current collector 211 has a foil-likeshape, a net-like shape, or the like so as to be flexible, a flexiblenegative electrode can be formed.

A method for forming the negative electrode 216 will be described withreference to FIGS. 6A to 6C. Here, as one mode of the active materiallayer 215, the active material layer 215 a illustrated in FIG. 5B willbe described.

As illustrated in FIG. 6A, a silicon layer 210 is formed over thecurrent collector 211. Then, as in Embodiment 1, masks 208 a to 208 eare formed over the silicon layer 210.

The silicon layer 210 can be formed by a CVD method, a sputteringmethod, or the like as appropriate. The silicon layer 210 is formedusing single crystal silicon, polycrystalline silicon, or amorphoussilicon. The silicon layer 210 may be formed using an n-type siliconlayer to which phosphorus is added or a p-type silicon layer to whichboron is added.

The silicon layer 210 is selectively etched with the use of the masks208 a to 208 e, so that the active material 212 is formed as illustratedin FIG. 6B. As a method for etching the silicon layer 210, a dry etchingmethod or a wet etching method can be used as appropriate. Note thatwhen a Bosch process which is a dry etching method is used, a highprotrusion can be formed.

After the masks 208 a to 208 e are removed, the graphene 214 is formedover the active material 212, so that the negative electrode 216 inwhich the active material layer 215 a is provided over the currentcollector 211 can be manufactured.

The graphene 214 can be formed in a manner similar to that of thegraphene 204 described in Embodiment 1.

Note that in FIG. 6B, when the common portion 212 a is etched to exposethe current collector 211, the negative electrode including the activematerial layer 215 b illustrated in FIG. 5C can be manufactured.

A protective layer is formed over the silicon layer 210, the masks 208 ato 208 e are formed over the protective layer, and separated protectivelayers 217 are formed with the use of the masks 208 a to 208 e (see FIG.5C). After that, with the use of the masks 208 a to 208 e and theseparated protective layers 217, the silicon layer 210 is selectivelyetched, whereby the negative electrode including the active materiallayer 215 c illustrated in FIG. 5D can be formed. When the plurality ofprotrusions 212 b are high, that is, the etching time is long, the masksare thinned gradually in the etching step and part of the masks areremoved to expose the silicon layer 210. Accordingly, there is variationin height between the protrusions. However, by using the separatedprotective layers 217 as hard masks, the silicon layer 210 can beprevented from being exposed so that variation in height between theprotrusions can be reduced.

Embodiment 3

In this embodiment, a negative electrode having a structure differentfrom those of Embodiments 1 and 2 and a method for manufacturing thenegative electrode will be described with reference to FIGS. 7A to 7C,FIGS. 8A to 8D, FIGS. 9A to 9D, FIGS. 10A to 10C, and FIGS. 11A and 11B.

FIG. 7A is a cross-sectional view of a negative electrode 266. Thenegative electrode 266 functions as an active material.

A specific structure of the negative electrode 266 will be describedwith reference to FIGS. 7B and 7C. Typical examples of the negativeelectrode 266 are a negative electrode 266 a and a negative electrode266 b in FIGS. 7B and 7C, respectively.

FIG. 7B is an enlarged cross-sectional view of the negative electrode266 a. The negative electrode 266 a includes an active material 262 anda graphene 264 provided over the active material 262. The activematerial 262 includes a common portion 262 a and a plurality ofprotrusions 262 b which protrude from the common portion 262 a. Thegraphene 264 covers at least part of the active material 262.Alternatively, the graphene 264 may cover surfaces of the common portion262 a and the plurality of protrusions 262 b of the active material 262.

The active material 262 can be formed using one or more materials givenfor the active material 202 in Embodiment 1.

The common portion 262 a serves as a base layer of the plurality ofprotrusions 262 b. The common portion 262 a is a continuous layer and iscontact with the plurality of protrusions 262 b.

As illustrated in FIGS. 7B and 7C, the protrusion 262 b has a shape inwhich the width of a bottom portion in contact with the common portion262 a is larger than the width of a top portion in a longitudinalcross-sectional shape. Each of the protrusions 262 b can have any of thefollowing shapes as appropriate: a columnar shape such as a cylindricalshape 281 (see FIG. 8A) or a prismatic shape, a conical shape 282 (seeFIG. 8B) or a pyramidal shape, a plate-like shape 283 (see FIG. 8C), apipe-like shape 284 (see FIG. 8D), and the like. Note that the top orthe edge of the protrusion 262 b may be curved. In FIG. 7B, acylindrical protrusion is used as the protrusion 262 b.

As described above, the protrusion 262 b has a shape in which the widthof the bottom portion in contact with the common portion 262 a is largerthan the width of the top portion in a longitudinal cross-sectionalshape. That is, each of the plurality of protrusions has a shape inwhich the bottom portion is wider than the top portion. Thus, themechanical strength is improved, and deterioration such as pulverizationor separation due to expansion and contraction of the active materialcaused by charge/discharge reaction can be suppressed. Further, assemblyof the battery is done with the use of the plurality of protrusions eachhaving a shape in which the bottom portion is wider than the top portionfor the negative electrode; in that case, even when the top portions ofthe plurality of protrusions are broken by being in contact with theseparator or the like, the bottom portions of the plurality ofprotrusions having high strength tend to remain. Accordingly, the yieldof the assembly to manufacture the battery can be improved.

A top view of the electrode in this embodiment will be described withreference to FIGS. 9A to 9D.

FIG. 9A is a top view illustrating the common portion 262 a and theplurality of protrusions 262 b which protrude from the common portion262 a. The plurality of protrusions 262 b which have circular top shapesare arranged. As illustrated in FIGS. 7B and 7C, the protrusion 262 bhas a shape in which the width of the bottom portion in contact with thecommon portion 262 a is larger than the width of the top portion in alongitudinal cross-sectional shape. Thus, in the top view, theprotrusion 262 b is denoted by two different circles. In thisembodiment, although the protrusion 262 b is denoted by two circleshaving different cross-sectional areas in the top shape, the presentinvention is not limited thereto, and the protrusion may be denoted bytwo or more circles having different cross-sectional areas. FIG. 9B is atop view after movement of the plurality of protrusions 262 b in FIG. 9Ain the direction a. In FIGS. 9A and 9B, the plurality of protrusions 262b are provided at the same positions. That is, the plurality ofprotrusions 262 b illustrated in FIG. 9A have translation symmetry.Here, the plurality of protrusions 262 b in FIG. 9A move in thedirection a; however, the same result as FIG. 9B can be obtained aftermovement in the direction b or c.

The proportion of the protrusion 262 b in the unit of symmetry which isdenoted by a dashed line 269 is preferably higher than or equal to 25%and lower than or equal to 60%. That is, the proportion of a space inthe unit of symmetry is preferably higher than or equal to 40% and lowerthan or equal to 75%. When the proportion of the protrusion 262 b in theunit of symmetry is higher than or equal to 25%, the theoreticalcharge/discharge capacity of the negative electrode can be higher thanor equal to about 1000 mAh/g. In addition, by setting the proportion ofthe protrusion 262 b in the unit of symmetry to lower than or equal to60%, also when the charge/discharge capacity is maximum (i.e.,theoretical capacity) and the protrusions expand, the adjacentprotrusions are not in contact with each other and can be prevented frombeing broken. As a result, high charge/discharge capacity can beobtained and deterioration of the negative electrode due tocharge/discharge can be reduced.

FIG. 9C is a top view illustrating the common portion 262 a and aplurality of protrusions which protrude from the common portion 262 a.The protrusion 262 b which has a circular top shape and a protrusion 262c which has a square top shape are alternately arranged. FIG. 9D is atop view after movement of the plurality of protrusions 262 b and 262 cin the direction b. In the top views of FIGS. 9C and 9D, the pluralityof protrusions 262 b and 262 c are provided at the same positions. Thatis, the plurality of protrusions 262 b and 262 c illustrated in FIG. 9Chave translation symmetry.

By providing the plurality of protrusions such that they havetranslation symmetry, variation in electron conductivity among theplurality of protrusions can be reduced. Accordingly, local reaction inthe positive electrode and the negative electrode can be reduced,reaction between carrier ions and the active material can occuruniformly, and diffusion overvoltage (concentration overvoltage) can beprevented, so that the reliability of battery characteristics can beincreased.

The common portion 262 a and the plurality of protrusions 262 b can havea single crystal structure or a polycrystalline structure asappropriate. Alternatively, the common portion 262 a can have a singlecrystal structure or a polycrystalline structure, and the plurality ofprotrusions 262 b can have an amorphous structure. Furtheralternatively, the common portion 262 a and part of the plurality ofprotrusions 262 b can have a single crystal structure or apolycrystalline structure, and the other part of the plurality ofprotrusions 262 b can have an amorphous structure. Note that the part ofthe plurality of protrusions 262 b includes at least a region in contactwith the common portion 262 a.

The interface between the common portion 262 a and the plurality ofprotrusions 262 b is not clear. Accordingly, in the active material 262,a plane including the deepest depression among depressions between theplurality of protrusions 262 b and parallel with a plane where theprotrusions 262 b are formed is defined as an interface 233 between thecommon portion 262 a and the plurality of protrusions 262 b.

In addition, the longitudinal directions of the plurality of protrusions262 b are oriented in the same direction. That is, axes 231 of theplurality of protrusions 262 b are oriented in the same direction.Preferably, the plurality of protrusions 262 b have substantially thesame shapes. With such a structure, the volume of the active materialcan be controlled. Further, the axis 231 of the protrusion is a straightline which passes the top of the protrusion (or the center of a topsurface of the protrusion) and the center of a bottom surface of theprotrusion which is in contact with the common portion. That is, theaxis is a straight line which passes the center of the longitudinaldirection of the protrusion. When the axes of the plurality ofprotrusions are oriented in the same direction, the axes of theplurality of protrusions are substantially parallel with each other.Specifically, the angle between the axes of the plurality of protrusionsis less than or equal to 10 degrees, preferably less than or equal to 5degrees.

The direction in which the plurality of protrusions 262 b extend fromthe common portion 262 a is referred to as a longitudinal direction, anda cross-sectional shape parallel with the longitudinal direction isreferred to as a longitudinal cross-sectional shape. In addition, across-sectional shape of a plane substantially perpendicular to thelongitudinal direction of the plurality of protrusions 262 b is referredto as a lateral cross-sectional shape.

The width of the bottom portion of the protrusion 262 b in a lateralcross-sectional shape is greater than or equal to 0.1 μm and less thanor equal to 1 μm, preferably greater than or equal to 0.2 μm and lessthan or equal to 0.5 μm. The height of the protrusion 262 b is fivetimes to hundred times, preferably ten times to fifty times of the widthof the bottom portion of the protrusion 262 b, typically, greater thanor equal to 0.5 μm and less than or equal to 100 μm, preferably greaterthan or equal to 1 μm and less than or equal to 50 μm.

With the width of the bottom portion of the protrusion 262 b in alateral cross-sectional shape being greater than or equal to 0.1 μm, thecharge/discharge capacity can be increased. With the width of the bottomportion of the protrusion 262 b in a lateral cross-sectional shape beingless than or equal to 1 μm, even when the plurality of protrusionsexpand in charge/discharge, the protrusions can be prevented from beingbroken. In addition, with the height of the protrusion 262 b beinggreater than or equal to 0.5 μm, the charge/discharge capacity can beincreased. With the height of the protrusion 262 b being less than orequal to 100 μm, even when the plurality of protrusions expand incharge/discharge, the protrusions can be prevented from being broken.

The height of the protrusion 262 b is a distance between the commonportion 262 a and the top (or the center of the top surface) of theprotrusion 262 b in the direction parallel with the axis which passesthe top, in a longitudinal cross-sectional shape.

The plurality of protrusions 262 b are provided over the common portion262 a with a predetermined distance therebetween. The distance betweenthe plurality of protrusions 262 b is preferably 1.29 times to 2 timesof the width of the bottom portion of the protrusion 262 b.Consequently, even when the volume of the protrusion 262 b increases dueto charge of the power storage device including the negative electrode,the protrusions 262 b are not in contact with each other and can beprevented from being broken, and moreover, a reduction incharge/discharge capacity of the power storage device can be prevented.

The graphene 264 functions as a conductive additive. The graphene 264functions as an active material in some cases. The graphene 204described in Embodiment 1 can be used as the graphene 264 asappropriate.

In addition, since the plurality of protrusions 262 b protrude from thecommon portion 262 a in the active material 262 of the negativeelectrode 266, the active material 262 has a larger surface area than aplate-like active material. Axes of the plurality of protrusions areoriented in the same direction and the protrusions protrude in thedirection perpendicular to the common portion, so that the density ofthe protrusions in the negative electrode can be increased and thesurface area thereof can be further increased. A space is providedbetween the plurality of protrusions. Further, a graphene covers theactive material. Thus, even when the active material expands incharging, contact between the protrusions can be reduced. Moreover, alsowhen the active material is separated, a graphene can prevent breakdownof the active material. The plurality of protrusions have translationsymmetry and formed with high uniformity in the negative electrode, sothat local reaction can be reduced in each of the positive electrode andthe negative electrode, and carrier ions and the active material canreact with each other uniformly between the positive electrode and thenegative electrode. Consequently, in the case where the negativeelectrode 266 is used for the power storage device, high-speedcharge/discharge becomes possible, and breakdown and separation of theactive material due to charge/discharge can be suppressed, whereby apower storage device with improved cycle characteristics can bemanufactured. Furthermore, when the shapes of the protrusions aresubstantially the same, local charge/discharge can be reduced, and theweight of the active material can be controlled. In addition, when theheights of the protrusions are substantially the same, load can beprevented from being applied locally in the manufacturing process of thebattery, which can increase the yield. Accordingly, specifications ofthe battery can be well controlled.

When the surface of the active material 262 is in contact with anelectrolyte in the power storage device, the electrolyte and the activematerial react with each other, so that a film is formed over a surfaceof the active material. The film is called a solid electrolyte interface(SEI) which is considered necessary for relieving reaction between theactive material and the electrolyte and for stabilization. However, whenthe film is thick, carrier ions are occluded by the active material withdifficulty, which might reduce the conductivity of carrier ions betweenthe active material and the electrolyte.

The graphene 264 covering the active material 262 can suppress anincrease in thickness of the film, so that a decrease in conductivity ofcarrier ions can be suppressed.

A graphene has high conductivity; by covering silicon with a graphene,electrons can transfer at sufficiently high speed in a graphene. Inaddition, a graphene has a thin sheet-like shape; by covering aplurality of protrusions with a graphene, the amount of the activematerial in the active material layer can be increased and carrier ionscan transfer more easily than in a graphite. As a result, theconductivity of carrier ions can be increased, reaction between siliconthat is an active material and carrier ions can be increased, andcarrier ions can be easily occluded by the active material. Accordingly,a power storage device including the negative electrode can performcharge/discharge at high speed.

Note that a silicon oxide layer may be provided between the activematerial 262 and the graphene 264 like the silicon oxide layer betweenthe active material 202 and the graphene 204 in Embodiment 1. In thatcase, side surfaces of the plurality of protrusion 262 b are in contactwith the graphene 264.

Like the negative electrode 266 b illustrated in FIG. 7C, a protectivelayer 277 may be provided between the top of the protrusion 262 b in theactive material 262 and the graphene 264.

The protective layer 277 can be formed in a manner similar to that ofthe protective layer 207 described in Embodiment 1.

Next, a method for manufacturing the negative electrode 266 will bedescribed with reference to FIGS. 10A to 10C and FIGS. 11A and 11B.Here, as one mode of the negative electrode 266, the negative electrode266 a illustrated in FIG. 7B will be described.

As illustrated in FIG. 10A, masks 268 a to 268 e are formed over asilicon substrate 260.

As the silicon substrate 260, the silicon substrate 200 described inEmbodiment 1 can be used as appropriate.

The masks 268 a to 268 e can be formed in a manner similar to that ofthe masks 208 a to 208 e described in Embodiment 1.

The silicon substrate 260 is selectively etched with the use of themasks 268 a to 268 e, so that an active material 261 is formed asillustrated in FIG. 10B. As a method for etching the silicon substrate260, an etching method used for etching the silicon substrate 200 can beused as appropriate.

Then, the masks 268 a to 268 e are reduced in size by oxygen plasmatreatment or the like, so that masks 268 f to 268 j are formed asillustrated in FIG. 10C.

With the use of the masks 268 f to 268 j, the active material 261 isselectively etched, so that as illustrated in FIG. 11A, the activematerial 262 including the common portion 262 a and the plurality ofprotrusions 262 b can be formed. In this embodiment, etching time isadjusted such that the common portion 262 a remains and the protrusion262 b has a shape in which the width of the bottom portion in contactwith the common portion 262 a is larger than the width of the topportion in a longitudinal cross-sectional shape. Etching of the activematerial 261 can be performed in a manner similar to that of the siliconsubstrate 260.

As described in this embodiment, the silicon substrate is etched withthe use of the masks, whereby the plurality of protrusions whose axesare oriented in the same direction can be formed. Further, the pluralityof protrusions whose shapes are substantially the same can be formed.

After the masks 268 f to 268 j are removed, the graphene 264 is formedover the active material 262, whereby the negative electrode 266 a asillustrated in FIG. 11B can be manufactured.

The graphene 264 can be formed by a method for forming the graphene 204described in Embodiment 1 as appropriate.

In accordance with this embodiment, the negative electrode 266 aillustrated in FIG. 7B can be formed.

The negative electrode 266 b illustrated in FIG. 7C can be formed insuch a manner that: a protective layer is formed over the siliconsubstrate 260, the masks 268 a to 268 e are formed over the protectivelayer, separated protective layers are formed with the use of the masks268 a to 268 e, the silicon substrate 260 is selectively etched with theuse of the masks 268 a to 268 e and the separated protective layers, themasks 268 a to 268 e are reduced in size by oxygen plasma treatment orthe like to form the masks 268 f to 268 j and protective layers 267 (seeFIG. 7C), and etching is further performed selectively with the use ofthe masks 268 f to 268 j and the separated protective layers 267. Whenthe plurality of protrusions 262 b are high, that is, the etching timeis long, the masks are thinned gradually in the etching step and part ofthe masks are removed to expose the silicon substrate 260. Accordingly,there is variation in height between the protrusions. However, by usingthe separated protective layers 267 as hard masks, the silicon substrate260 can be prevented from being exposed so that variation in heightbetween the protrusions can be reduced.

Embodiment 4

In this embodiment, a negative electrode having a structure differentfrom those of Embodiments 1 to 3 and a method for manufacturing thenegative electrode will be described with reference to FIGS. 12A to 12D,FIGS. 13A to 13C, and FIGS. 14A and 14B. The negative electrodedescribed in this embodiment is different from those of Embodiments 1 to3 in that a current collector is provided. Further, the shape of theprotrusion is different from that in Embodiment 2.

FIG. 12A is a cross-sectional view of a negative electrode 276. In thenegative electrode 276, an active material layer 275 is provided over acurrent collector 271.

A specific structure of the negative electrode 276 will be describedwith reference to FIGS. 12B to 12D. Typical examples of the activematerial layer 275 included in the negative electrode 276 are an activematerial layer 275 a, an active material layer 275 b, and an activematerial layer 275 c in FIGS. 12B, 12C, and 12D, respectively.

FIG. 12B is an enlarged cross-sectional view of the current collector271 and the active material layer 275 a. The active material layer 275 ais provided over the current collector 271. The active material layer275 a includes an active material 272 and a graphene 274 provided overthe active material 272. The active material 272 includes a commonportion 272 a and a plurality of protrusions 272 b which protrude fromthe common portion 272 a. In addition, the longitudinal directions ofthe plurality of protrusions 272 b are oriented in the same direction.That is, axes 241 of the plurality of protrusions 272 b are oriented inthe same direction.

As illustrated in FIGS. 12B to 12D, the protrusion 272 b has a shape inwhich the width of a bottom portion in contact with the common portionor the current collector is larger than the width of a top portion in alongitudinal cross-sectional shape.

As described above, the protrusion 272 b has a shape in which the widthof the bottom portion in contact with the common portion or the currentcollector is larger than the width of the top portion in a longitudinalcross-sectional shape. That is, each of the plurality of protrusions hasa shape in which the bottom portion is wider than the top portion. Thus,the mechanical strength is improved, and deterioration such aspulverization or separation due to expansion and contraction of theactive material caused by charge/discharge reaction can be suppressed.Further, assembly of the battery is done with the use of the pluralityof protrusions each having a shape in which the bottom portion is widerthan the top portion for the negative electrode; in that case, even whenthe top portions of the plurality of protrusions are broken by being incontact with the separator or the like, the bottom portions of theplurality of protrusions having high strength tend to remain.Accordingly, the yield of the assembly to manufacture the battery can beimproved.

As the current collector 271, the current collector 211 described inEmbodiment 2 can be used as appropriate.

The active material 272 can be formed using a material similar to thatof the active material 202 in Embodiment 1.

The common portion 272 a is a layer which serves as a base layer of theplurality of protrusions 272 b and is continuous over the currentcollector 271, similarly to the common portion 262 a in Embodiment 3. Inaddition, the common portion 272 a and the plurality of protrusions 272b are in contact with each other.

The plurality of protrusions 272 b can have the same shape as theplurality of protrusions 262 b in Embodiment 3 as appropriate.

The common portion 272 a and the plurality of protrusions 272 b can havea single crystal structure, a polycrystalline structure, or an amorphousstructure as appropriate. In addition, the common portion 272 a and theplurality of protrusions 272 b can have a crystalline structure which isintermediate of these structures, such as a microcrystalline structure.Alternatively, the common portion 272 a can have a single crystalstructure or a polycrystalline structure, and the plurality ofprotrusions 272 b can have an amorphous structure. Furtheralternatively, the common portion 272 a and part of the plurality ofprotrusions 272 b can have a single crystal structure or apolycrystalline structure, and the other part of the plurality ofprotrusions 272 b can have an amorphous structure. Note that the part ofthe plurality of protrusions 272 b includes at least a region in contactwith the common portion 272 a.

The width or height of the protrusion 272 b can be the same as theprotrusion 262 b in Embodiment 3.

As the graphene 274, a graphene having a structure similar to that ofthe graphene 264 in Embodiment 3 can be used as appropriate.

Like the active material layer 275 b in FIG. 12C, the negative electrode276 may have a structure in which the common portion is not provided,the plurality of protrusions 272 b which are separated from each otherare provided over the current collector 271, and the graphene 274 isformed over the current collector 271 and the plurality of protrusions272 b. Axes 251 of the plurality of protrusions 272 b are oriented inthe same direction.

The graphene 274 is in contact with part of the current collector 271,so that electrons can flow easily in the graphene 274 and reactionbetween the carrier ions and the active material can be improved.

Like the active material layer 275 c illustrated in FIG. 12D, aprotective layer 277 may be provided between the top of the protrusion272 b and the graphene 274. A material similar to that for theprotective layer 207 described in Embodiment 1 can be used for theprotective layer 277 as appropriate. Description is given using theactive material 272 in FIG. 12B here, but the protective layer 277 maybe provided over the active material in FIG. 12C.

In the negative electrode described in this embodiment, the activematerial layer can be provided using the current collector 271 as asupport. Accordingly, when the current collector 271 has a foil-likeshape, a net-like shape, or the like so as to be flexible, a flexiblenegative electrode can be formed.

A method for forming the negative electrode 276 will be described withreference to FIGS. 13A to 13C and FIGS. 14A and 14B. Here, as one modeof the active material layer 275, the active material layer 275 aillustrated in FIG. 12B will be described.

As illustrated in FIG. 13A, a silicon layer 270 is formed over thecurrent collector 271 as in Embodiment 2. Then, as in Embodiment 1,masks 268 a to 268 e are formed over the silicon layer 270.

The silicon layer 270 is selectively etched with the use of the masks268 a to 268 e, so that an active material 280 is formed as illustratedin FIG. 13B. As a method for etching the silicon layer 270, an etchingmethod described in Embodiment 1 can be used as appropriate.

Then, the masks 268 a to 268 e are reduced in size by oxygen plasmatreatment or the like, so that masks 268 f to 268 j are formed asillustrated in FIG. 13C.

With the use of the masks 268 f to 268 j, the active material 280 isselectively etched, so that as illustrated in FIG. 14A, the activematerial 272 including the common portion 272 a and the plurality ofprotrusions 272 b can be formed. In this embodiment, etching time isadjusted such that the common portion 272 a remains and the protrusion272 has a shape in which the width of the bottom portion in contact withthe common portion 272 a is larger than the width of the top portion ina longitudinal cross-sectional shape. Etching of the active material 280can be performed in a manner similar to that of the etching of thesilicon layer 270.

After the masks 268 f to 268 j are removed, the graphene 274 is formedover the active material 272, so that as illustrated in FIG. 14B, thenegative electrode in which the active material layer 275 a is providedover the current collector 271 can be manufactured.

The graphene 274 can be formed in a manner similar to that of thegraphene 204 described in Embodiment 1.

Note that in FIG. 14A, when the common portion 272 a is etched to exposethe current collector 271, the negative electrode including the activematerial layer 275 b illustrated in FIG. 12C can be manufactured.

The negative electrode illustrated in FIG. 12D can be formed in such amanner that: a protective layer is formed over the silicon layer 270,the masks 268 a to 268 e are formed over the protective layer, separatedprotective layers are formed with the use of the masks 268 a to 268 e,the silicon layer 270 is selectively etched with the use of the masks268 a to 268 e and the separated protective layers, the masks 268 a to268 e are reduced in size by oxygen plasma treatment or the like to formthe masks 268 f to 268 j and protective layers 277 (see FIG. 12D), andetching is further performed selectively with the use of the masks 268 fto 268 j and the separated protective layers 277. When the plurality ofprotrusions 272 b are high, that is, the etching time is long, the masksare thinned gradually in the etching step and part of the masks areremoved to expose the silicon layer 270. Accordingly, there is variationin height between the protrusions. However, by using the separatedprotective layers 277 as hard masks, the silicon layer 270 can beprevented from being exposed so that variation in height between theprotrusions can be reduced.

Embodiment 5

In this embodiment, a negative electrode having a structure differentfrom those of Embodiments 1 to 4 and a method for manufacturing thenegative electrode will be described with reference to FIGS. 15A to 15C,FIGS. 16A to 16C, and FIGS. 17A to 17C. In this embodiment, descriptionis made using Embodiment 1; however, this embodiment can also be appliedto Embodiment 3 as appropriate.

FIG. 15A is a cross-sectional view of a negative electrode 206. Thenegative electrode 206 functions as an active material. An insulatinglayer functioning as a spacer (hereinafter referred to as a spacer 209)is provided over the negative electrode 206.

A specific structure of the negative electrode 206 will be describedwith reference to FIGS. 15B and 15C. Typical examples of the negativeelectrode 206 are a negative electrode 206 a and a negative electrode206 b in FIGS. 15B and 15C, respectively.

FIG. 15B is an enlarged cross-sectional view of the negative electrode206 a and the spacer 209. The negative electrode 206 a includes anactive material 202 and a graphene 204 provided over the active material202. The active material 202 includes a common portion 202 a and aplurality of protrusions 202 b which protrude from the common portion202 a. The spacer 209 is provided over the graphene 204 of the negativeelectrode 206 a.

The spacer 209 has an insulating property and is formed using a materialwhich does not react with an electrolyte. Specifically, an organicmaterial such as an acrylic resin, an epoxy resin, a silicone resin,polyimide, or polyamide, a low-melting-point glass material such asglass paste, glass frit, or glass ribbon, or the like can be used. Notethat a solute of an electrolyte described later may be mixed into thematerial which has an insulating property and does not react with anelectrolyte. As a result, the spacer 209 also functions as a solidelectrolyte.

The thickness of the spacer 209 is preferably greater than or equal to 1μm and less than or equal to 10 μm, more preferably greater than orequal to 2 μm and less than or equal to 7 μm. As a result, as comparedto the case where a separator having a thickness of several tens ofmicrometers is provided between the positive electrode and the negativeelectrode as in a conventional power storage device, the distancebetween the positive electrode and the negative electrode can bereduced, and the distance of movement of carrier ions between thepositive electrode and the negative electrode can be short. Accordingly,the positive electrode and the negative electrode can be prevented frombeing in contact with each other, and carrier ions included in the powerstorage device can be effectively used for charge/discharge.

The shape of the spacer 209 is described with reference to FIGS. 16A to16C. FIGS. 16A to 16C are top views of the negative electrode 206.Typical examples of the spacer 209 are a spacer 209 a, a spacer 209 b,and a spacer 209 c in FIGS. 16A, 16B, and 16C, respectively. In FIGS.16A to 16C, the plurality of protrusions 202 b are denoted by dashedlines and the spacers 209 a to 209 c are denoted by solid lines.

FIG. 16A is a top view illustrating the negative electrode 206 in whichone spacer 209 a is provided over each protrusion 202 b. Here, the shapeof the spacer 209 a is circular, but may also be polygonal asappropriate.

FIG. 16B is a top view illustrating the negative electrode 206 in whichthe rectangular spacer 209 b is provided over the protrusions 202 b.Here, one spacer 209 b is provided so as to form a straight band overthe plurality of protrusions 202 b. Note that in this embodiment, theside surface of the spacer 209 b is straight but may also be curved.

FIG. 16C is a top view illustrating the negative electrode 206 in whichthe lattice-like spacer 209 c is provided over the protrusions 202 b.Here, one spacer 209 c is provided over the plurality of protrusions 202b.

The shape of the spacer 209 is not limited those in FIGS. 16A to 16C. Itis sufficient as long as the spacer 209 has an opening in part of it andmay have a closed circular or polygonal loop shape.

Since the spacer 209 is provided over the negative electrode 206, aseparator is not needed in the power storage device completed later.Consequently, the number of components of the power storage device andthe cost can be reduced.

Like the negative electrode 206 b illustrated in FIG. 15C, a protectivelayer 207 may be provided between the top of the protrusion 202 b in theactive material 202 and the graphene 204.

Next, a method for manufacturing the negative electrode 206 will bedescribed with reference to FIGS. 17A to 17C. Here, as one mode of thenegative electrode 206, the negative electrode 206 a illustrated in FIG.15B will be described.

As illustrated in FIG. 17A, masks 208 a to 208 e are formed over asilicon substrate 200 as in Embodiment 1.

The silicon substrate 200 is selectively etched with the use of themasks 208 a to 208 e as in Embodiment 1, so that the active material 202is formed as illustrated in FIG. 17B.

Next, after the masks 208 a to 208 e are removed, the graphene 204 isformed over the active material 202 as in Embodiment 1, so that thenegative electrode 206 a can be formed as illustrated in FIG. 17C.

Then, the spacer 209 is formed over the graphene 204 (see FIG. 17C). Thespacer 209 can be formed in such a manner that a composition containinga material of the spacer 209 is selectively provided over theprotrusions by a printing method, an inkjet method, or the like andheated so as to vaporize a solvent of the composition containing amaterial of the spacer 209. Alternatively, the spacer 209 can be formedin such a manner that only top portions of the protrusions are soaked inthe composition containing a material of the spacer 209, and thenheating is performed so as to vaporize a solvent of the compositioncontaining a material of the spacer 209.

In accordance with this embodiment, the negative electrode 206 aillustrated in FIG. 15B can be formed.

A protective layer is formed over the silicon substrate 200, the masks208 a to 208 e are formed over the protective layer, and separatedprotective layers 207 are formed with the use of the masks 208 a to 208e (see FIG. 15C). After that, with the use of the masks 208 a to 208 eand the separated protective layers, the silicon substrate 200 isselectively etched. After that, the graphene 204 and the spacer 209 areformed, whereby the negative electrode 206 b in FIG. 15C can be formed.When the plurality of protrusions 202 b are high, that is, the etchingtime is long, the masks are thinned gradually in the etching step andpart of the masks are removed to expose the silicon substrate 200.Accordingly, there is variation in height between the protrusions.However, by using the separated protective layers 207 as hard masks, thesilicon substrate 200 can be prevented from being exposed so thatvariation in height between the protrusions can be reduced.

Embodiment 6

In this embodiment, a negative electrode having a structure differentfrom those of Embodiments 1 to 5 and a method for manufacturing thenegative electrode will be described with reference to FIGS. 18A to 18Dand FIGS. 19A to 19C. The negative electrode described in thisembodiment is different from that of Embodiment 5 in that a currentcollector is provided. In this embodiment, description is made usingEmbodiment 2; however, this embodiment can also be applied to Embodiment4 as appropriate.

FIG. 18A is a cross-sectional view of a negative electrode 216. In thenegative electrode 216, an active material layer 215 is provided over acurrent collector 211. An insulating layer functioning as a spacer(hereinafter referred to as a spacer 219) is provided over the negativeelectrode 216.

A specific structure of the negative electrode 216 will be describedwith reference to FIGS. 18B to 18C. Typical examples of the activematerial layer 215 included in the negative electrode 216 are an activematerial layer 215 a, an active material layer 215 b, and an activematerial layer 215 c in FIGS. 18B, 18C, and 18D, respectively.

FIG. 18B is an enlarged cross-sectional view of the current collector211, the active material layer 215 a, and the spacer 219. The activematerial layer 215 a is provided over the current collector 211. Thespacer 219 is provided over a graphene 214 of the active material layer215 a.

The spacer 219 can be formed using a material similar to that of thespacer 209 in Embodiment 5.

Like the active material layer 215 b in FIG. 18C, the negative electrode216 may have a structure in which the common portion is not provided,the plurality of protrusions 212 b which are separated from each otherare provided over the current collector 211, and the graphene 214 isformed over the current collector 211 and the plurality of protrusions212 b.

The graphene 214 is in contact with part of the current collector 211,so that electrons can flow easily in the graphene 214 and reactionbetween the carrier ions and the active material can be improved.

Like the active material layer 215 c illustrated in FIG. 18D, aprotective layer 217 may be provided between the top of the protrusion212 b and the graphene 214.

A method for forming the negative electrode 216 will be described withreference to FIGS. 19A to 19C. Here, as one mode of the active materiallayer 215, the active material layer 215 a illustrated in FIG. 18B willbe described.

As illustrated in FIG. 19A, a silicon layer 210 is formed over thecurrent collector 211 as in Embodiment 2. Then, as in Embodiment 2,masks 208 a to 208 e are formed over the silicon layer 210.

The silicon layer 210 is selectively etched with the use of the masks208 a to 208 e as in Embodiment 1, so that the active material 212 isformed as illustrated in FIG. 19B.

Next, as in Embodiment 1, after the masks 208 a to 208 e are removed,the graphene 214 is formed over the active material 212.

The graphene 214 can be formed in a manner similar to that of thegraphene 204 described in Embodiment 5.

Then, the spacer 219 is formed over the graphene 214 (see FIG. 19C).

The spacer 219 can be formed in a manner similar to that of the spacer209 in Embodiment 5.

Through the above steps, the negative electrode 216 in which the activematerial layer 215 a is provided over the current collector 211 and thespacer 219 can be manufactured.

Note that in FIG. 19B, when the common portion 212 a is etched to exposethe current collector 211, the negative electrode including the activematerial layer 215 b illustrated in FIG. 18C can be manufactured.

A protective layer is formed over the silicon layer 210, the masks 208 ato 208 e are formed over the protective layer, and separated protectivelayers 217 are formed with the use of the masks 208 a to 208 e (see FIG.18C). After that, with the use of the masks 208 a to 208 e and theseparated protective layers, the silicon layer 210 is selectivelyetched. After that, the graphene 214 and the spacer 219 are formed,whereby the negative electrode including the active material layer 215 cas illustrated in FIG. 18D can be formed. When the plurality ofprotrusions 212 b are high, that is, the etching time is long, the masksare thinned gradually in the etching step and part of the masks areremoved to expose the silicon layer 210. Accordingly, there is variationin height between the protrusions. However, by using the separatedprotective layers 217 as hard masks, the silicon layer 210 can beprevented from being exposed so that variation in height between theprotrusions can be reduced.

Embodiment 7

In this embodiment, a structure of a power storage device and amanufacturing method of the power storage device will be described.

First, a positive electrode and a manufacturing method thereof will bedescribed.

FIG. 20A is a cross-sectional view of a positive electrode 311. In thepositive electrode 311, a positive electrode active material layer 309is formed over a positive electrode current collector 307.

As the positive electrode current collector 307, a material having highconductivity such as platinum, aluminum, copper, titanium, or stainlesssteel can be used. The positive electrode current collector 307 can havea foil-like shape, a plate-like shape, a net-like shape, or the like asappropriate.

The positive electrode active material layer 309 can be formed using, asa material, a lithium compound such as LiFeO₂, LiCoO₂, LiNiO₂, orLiMn₂O₄, or V₂O₅, Cr₂O₅, MnO₂, or the like.

Alternatively, an olivine-type lithium-containing composite oxide (ageneral formula LiMPO₄ (M is one or more of Fe, Mn, Co, and Ni)) can beused. Typical examples of the general formula LiMPO₄ which can be usedas a material are lithium compounds such as LiFePO₄, LiNiPO₄, LiCoPO₄,LiMnPO₄, LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄,LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≦1, 0<a<1, and 0<b<1),LiFe_(e)Ni_(d)Co_(e)PO₄, LiFe_(e)Ni_(d)Mn_(e)PO₄,LiNi_(e)Co_(d)Mn_(e)PO₄ (c+d+e≦1, 0<c<1, 0<d<1, and 0<e<1), andLiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≦1, 0<f<1, 0<g<1, 0<h<1, and0<i<1).

Alternatively, a lithium-containing composite oxide such as a generalformula Li₂MSiO₄ (M is one or more of Fe, Mn, Co, and Ni) may be used.Typical examples of the general formula Li₂MSiO₄ which can be used as amaterial are lithium compounds such as Li₂FeSiO₄, Li₂NiSiO₄, Li₂CoSiO₄,Li₂MnSiO₄, Li₂Fe_(k)Ni_(l)SiO₄, Li₂Fe_(k)Co_(l)SiO₄,Li₂Fe_(k)Mn_(l)SiO₄, Li₂Ni_(k)Co_(l)SiO₄, Li₂Ni_(k)Mn_(l)SiO₄ (k+l≦1,0<k<1, and 0<l<1), Li₂Fe_(m)Ni_(n)Co_(q)SiO₄, Li₂Fe_(m)Ni_(n)Mn_(q)SiO₄,Li₂Ni_(m)Co_(n)Mn_(q)SiO₄ (m+n+q≦1, 0<m<1, 0<n<1, and 0<q<1), andLi₂Fe_(r)Ni_(s)Co_(t)Mn_(u)SiO₄ (r+s+t+u≦1, 0<r<1, 0<s<1, 0<t<1, and0<u<1).

In the case where carrier ions are alkali metal ions other than lithiumions, alkaline-earth metal ions, beryllium ions, or magnesium ions, thepositive electrode active material layer 309 may contain, instead oflithium in the lithium compound and the lithium-containing compositeoxide, an alkali metal (e.g., sodium or potassium), an alkaline-earthmetal (e.g., calcium, strontium, or barium), beryllium, or magnesium.

FIG. 20B is a plan view of the positive electrode active material layer309. The positive electrode active material layer 309 contains positiveelectrode active materials 321 which are particles capable of occludingand releasing carrier ions, and graphenes 323 which cover a plurality ofparticles of the positive electrode active materials 321 and at leastpartly surround the plurality of particles of the positive electrodeactive materials 321. The plurality of graphenes 323 cover surfaces ofthe plurality of particles of the positive electrode active materials321. The positive electrode active materials 321 may partly be exposed.The graphene 204 described in Embodiment 1 can be used as the graphene323 as appropriate.

The size of the particle of the positive electrode active material 321is preferably greater than or equal to 20 nm and less than or equal to100 nm. Note that the size of the particle of the positive electrodeactive material 321 is preferably smaller because electrons transfer inthe positive electrode active materials 321.

In the case where the positive electrode active material layer 309contains the graphenes 323, sufficient characteristics can be obtainedeven when surfaces of the positive electrode active materials 321 arenot covered with a graphite layer; however, it is preferable to use boththe graphene 323 and the positive electrode active material covered witha graphite layer because electrons transfer hopping between the positiveelectrode active materials and current flows.

FIG. 20C is a cross-sectional view of part of the positive electrodeactive material layer 309 in FIG. 20B. The positive electrode activematerial layer 309 includes the positive electrode active materials 321and the graphenes 323 which partly cover the positive electrode activematerials 321. The graphenes 323 are observed to have linear shapes incross section. A plurality of particles of the positive electrode activematerials are at least partly surrounded with one graphene or pluralgraphenes. That is, the plurality of particles of the positive electrodeactive materials exist within one graphene or among plural graphenes.Note that the graphene has a bag-like shape, and the plurality particlesof the positive electrode active materials are at least partlysurrounded with the bag-like portion in some cases. In addition, thepositive electrode active materials are not covered with the graphenesand partly exposed in some cases.

The desired thickness of the positive electrode active material layer309 is determined in the range of greater than or equal to 20 μm andless than or equal to 100 μm. It is preferable to adjust the thicknessof the positive electrode active material layer 309 as appropriate sothat a crack and separation are not caused.

Note that the positive electrode active material layer 309 may containacetylene black particles having a volume 0.1 times to 10 times as largeas that of the graphene, carbon particles having a one-dimensionalexpansion (e.g., carbon nanofibers), or other known binders.

As an example of the positive electrode active material, a materialwhose volume is expanded by occlusion of ions serving as carriers isgiven. When such a material is used, the positive electrode activematerial layer gets vulnerable and is partly broken by charge/discharge,resulting in lower reliability of a power storage device. However, thegraphene 323 covering the periphery of the positive electrode activematerials allows prevention of dispersion of the positive electrodeactive materials and the breakdown of the positive electrode activematerial layer, even when the volume of the positive electrode activematerials is increased and decreased due to charge/discharge. That is tosay, the graphene has a function of maintaining the bond between thepositive electrode active materials even when the volume of the positiveelectrode active materials is increased and decreased bycharge/discharge.

The graphene 323 is in contact with a plurality of particles of thepositive electrode active materials and also serves as a conductiveadditive. Further, the graphene 323 has a function of holding thepositive electrode active materials 321 capable of occluding andreleasing carrier ions. Thus, a binder does not have to be mixed intothe positive electrode active material layer. Accordingly, theproportion of the positive electrode active materials in the positiveelectrode active material layer can be increased and the dischargecapacity of a power storage device can be increased.

Next, a manufacturing method of the positive electrode active materiallayer 309 will be described.

Slurry containing particles of positive electrode active materials andgraphene oxide is formed. After a positive electrode current collectoris coated with the slurry, heating is performed in a reducing atmospherefor reduction treatment so that the positive electrode active materialsare baked and part of oxygen is released from graphene oxide to formopenings in graphene, as in the manufacturing method of graphene, whichis described in Embodiment 1. Note that oxygen in the graphene oxide isnot entirely reduced and partly remains in the graphene. Through theabove process, the positive electrode active material layer 309 can beformed over the positive electrode current collector 307. Consequently,the positive electrode active material layer 309 has higherconductivity.

Graphene oxide contains oxygen and thus is negatively charged in a polarsolvent. As a result of being negatively charged, graphene oxide isdispersed. Accordingly, the positive electrode active materialscontained in the slurry are not easily aggregated, so that the size ofthe particle of the positive electrode active material can be preventedfrom increasing by baking. Thus, the transfer of electrons in thepositive electrode active materials is facilitated, resulting in anincrease in conductivity of the positive electrode active materiallayer.

As illustrated in FIGS. 21A and 21B, a spacer 331 may be provided over asurface of the positive electrode 311. FIG. 21A is a perspective view ofthe positive electrode including the spacer, and FIG. 21B is across-sectional view along dashed and dotted line A-B in FIG. 21A.

As illustrated in FIGS. 21A and 21B, in the positive electrode 311, thepositive electrode active material layer 309 is provided over thepositive electrode current collector 307. The spacer 331 is providedover the positive electrode active material layer 309.

The spacer 331 has an insulating property and can be formed using amaterial which does not react with an electrolyte. Specifically, anorganic material such as an acrylic resin, an epoxy resin, a siliconeresin, polyimide, or polyamide, low-melting-point glass such as glasspaste, glass frit, or glass ribbon, or the like can be used. Since thespacer 331 is provided over the positive electrode 311, a separator isnot needed in the power storage device completed later. Consequently,the number of components of the power storage device and the cost can bereduced.

The spacer 331 preferably has a planar shape which exposes part of thepositive electrode active material layer 309, such as lattice-like shapeor a closed circular or polygonal loop shape. As a result, contactbetween the positive electrode and the negative electrode can beprevented, and the transfer of carrier ions between the positiveelectrode and the negative electrode can be promoted.

The thickness of the spacer 331 is preferably greater than or equal to 1μm and less than or equal to 5 μm, preferably greater than or equal to 2μm and less than or equal to 3 μm. As a result, as compared to the casewhere a separator having a thickness of several tens of micrometers isprovided between the positive electrode and the negative electrode as ina conventional power storage device, the distance between the positiveelectrode and the negative electrode can be reduced, and the distance ofmovement of carrier ions between the positive electrode and the negativeelectrode can be short. Accordingly, carrier ions included in the powerstorage device can be effectively used for charge/discharge.

The spacer 331 can be formed by a printing method, an ink jettingmethod, or the like as appropriate.

Next, a structure of a power storage device and a manufacturing methodthereof will be described.

An embodiment of a lithium-ion secondary battery in this embodimentwhich is a typical example of power storage devices will be describedwith reference to FIG. 22. Here, description is made below on across-sectional structure of the lithium-ion secondary battery.

FIG. 22 is a cross-sectional view of the lithium-ion secondary battery.

A lithium-ion secondary battery 400 includes a negative electrode 411including a negative electrode current collector 407 and a negativeelectrode active material layer 409, a positive electrode 405 includinga positive electrode current collector 401 and a positive electrodeactive material layer 403, and a separator 413 provided between thenegative electrode 411 and the positive electrode 405. Note that theseparator 413 includes an electrolyte 415. The negative electrodecurrent collector 407 is connected to an external terminal 419 and thepositive electrode current collector 401 is connected to an externalterminal 417. An end portion of the external terminal 419 is embedded ina gasket 421. In other words, the external terminals 417 and 419 areinsulated from each other with the gasket 421.

The negative electrode 206 described in Embodiment 1, the negativeelectrode 216 described in Embodiment 2, the negative electrode 266described in Embodiment 3, or the negative electrode 276 described inEmbodiment 4 can be used as appropriate as the negative electrode 411.

As the positive electrode current collector 401 and the positiveelectrode active material layer 403, the positive electrode currentcollector 307 and the positive electrode active material layer 309 whichare described in this embodiment can be used as appropriate.

An insulating porous material is used for the separator 413. Typicalexamples of the separator 413 include cellulose (paper), polyethylene,and polypropylene.

When a positive electrode including a spacer over a positive electrodeactive material layer as illustrated in FIGS. 21A and 21B is used as thepositive electrode 405, the separator 413 is not necessarily provided.

As a solute of the electrolyte 415, a material including carrier ions isused. Typical examples of the solute of the electrolyte include lithiumsalt such as LiClO₄, LiAsF₆, LiBF₄, LiPF₆, and Li(C₂F₅SO₂)₂N.

Note that when carrier ions are alkali metal ions other than lithiumions, alkaline-earth metal ions, beryllium ions, or magnesium ions,instead of lithium in the above lithium salts, an alkali metal (e.g.,sodium or potassium), an alkaline-earth metal (e.g., calcium, strontium,or barium), beryllium, or magnesium may be used for a solute of theelectrolyte 415.

As a solvent of the electrolyte 415, a material in which carrier ionscan transfer is used. As the solvent of the electrolyte 415, an aproticorganic solvent is preferably used. Typical examples of an aproticorganic solvent include ethylene carbonate, propylene carbonate,dimethyl carbonate, diethyl carbonate, γ-butyrolactone, acetonitrile,dimethoxyethane, tetrahydrofuran, and the like, and one or more of thesematerials can be used. When a gelled polymer material is used as thesolvent of the electrolyte 415, safety against liquid leakage or thelike is increased. Further, the lithium-ion secondary battery 400 can bethinner and more lightweight. Typical examples of a gelled polymermaterial include a silicon gel, an acrylic gel, an acrylonitrile gel,polyethylene oxide, polypropylene oxide, a fluorine-based polymer, andthe like. In addition, by using one or plural kinds of ionic liquid(room-temperature molten salt) which has features of non-flammabilityand non-volatility as a solvent of the electrolyte 415, short-circuitinside the power storage device can be prevented, and moreover, evenwhen the internal temperature is increased due to overcharge or thelike, explosion, ignition, or the like of the power storage device canbe prevented.

As the electrolyte 415, a solid electrolyte such as Li₃PO₄ can be used.Note that in the case of using the solid electrolyte as the electrolyte415, the separator 413 is unnecessary.

For the external terminals 417 and 419, a metal member such as astainless steel plate or an aluminum plate can be used as appropriate.

Note that in this embodiment, a coin-type lithium-ion secondary batteryis given as the lithium-ion secondary battery 400; however, any oflithium-ion secondary batteries with various shapes, such as asealing-type lithium-ion secondary battery, a cylindrical lithium-ionsecondary battery, and a square-type lithium-ion secondary battery, canbe used. Further, a structure in which a plurality of positiveelectrodes, a plurality of negative electrodes, and a plurality ofseparators are stacked or rolled may be employed.

Next, a method for manufacturing the lithium-ion secondary battery 400described in this embodiment will be described.

By the manufacturing method described in Embodiment 1 and thisembodiment, the positive electrode 405 and the negative electrode 411are formed as appropriate.

Next, the positive electrode 405, the separator 413, and the negativeelectrode 411, are impregnated with the electrolyte 415. Then, thepositive electrode 405, the separator 413, the gasket 421, the negativeelectrode 411, and the external terminal 419 are stacked in this orderover the external terminal 417, and the external terminal 417 and theexternal terminal 419 are crimped to each other with a “coin cellcrimper”. Thus, the coin-type lithium ion secondary battery can bemanufactured.

Note that a spacer and a washer may be provided between the externalterminal 417 and the positive electrode 405 or between the externalterminal 419 and the negative electrode 411 so that connection betweenthe external terminal 417 and the positive electrode 405 or between theexternal terminal 419 and the negative electrode 411 is enhanced.

Embodiment 8

A power storage device according to an embodiment of the presentinvention can be used as a power supply of various electric deviceswhich are driven by electric power.

Specific examples of electric devices using the power storage deviceaccording to an embodiment of the present invention are as follows:display devices, lighting devices, desktop personal computers or laptoppersonal computers, image reproduction devices which reproduce a stillimage or a moving image stored in a recording medium such as a digitalversatile disc (DVD), mobile phones, portable game machines, portableinformation terminals, e-book readers, cameras such as video cameras anddigital still cameras, high-frequency heating apparatuses such asmicrowaves, electric rice cookers, electric washing machines,air-conditioning systems such as air conditioners, electricrefrigerators, electric freezers, electric refrigerator-freezers,freezers for preserving DNA, dialysis devices, and the like. Inaddition, moving objects driven by an electric motor using power from apower storage device are also included in the category of electricdevices. As examples of the moving objects, electric vehicles, hybridvehicles which include both an internal-combustion engine and anelectric motor, motorized bicycles including motor-assisted bicycles,and the like can be given.

In the electric devices, the power storage device according to anembodiment of the present invention can be used as a power storagedevice for supplying power for almost the whole power consumption (sucha power storage device is referred to as a main power supply).Alternatively, in the electric devices, the power storage deviceaccording to an embodiment of the present invention can be used as apower storage device which can supply power to the electric devices whenthe supply of power from the main power supply or a commercial powersupply is stopped (such a power storage device is referred to as anuninterruptible power supply). Further alternatively, in the electricdevices, the power storage device according to an embodiment of thepresent invention can be used as a power storage device for supplyingpower to the electric devices at the same time as the power supply fromthe main power supply or a commercial power supply (such a power storagedevice is referred to as an auxiliary power supply).

FIG. 23 illustrates specific structures of the electric devices. In FIG.23, a display device 5000 is an example of an electric device includinga power storage device 5004 according to an embodiment of the presentinvention. Specifically, the display device 5000 corresponds to adisplay device for TV broadcast reception and includes a housing 5001, adisplay portion 5002, speaker portions 5003, the power storage device5004, and the like. The power storage device 5004 according to oneembodiment of the present invention is provided inside the housing 5001.The display device 5000 can receive power from a commercial powersupply. Alternatively, the display device 5000 can use power stored inthe power storage device 5004. Thus, the display device 5000 can beoperated with the use of the power storage device 5004 according to anembodiment of the present invention as an uninterruptible power supplyeven when power cannot be supplied from a commercial power supply due topower failure or the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), a field emission display (FED), and the like can be used for thedisplay portion 5002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like other than TV broadcast reception.

In FIG. 23, an installation lighting device 5100 is an example of anelectric device including a power storage device 5103 according to anembodiment of the present invention. Specifically, the lighting device5100 includes a housing 5101, a light source 5102, the power storagedevice 5103, and the like. FIG. 23 shows the case where the powerstorage device 5103 is provided in a ceiling 5104 on which the housing5101 and the light source 5102 are installed; alternatively, the powerstorage device 5103 may be provided in the housing 5101. The lightingdevice 5100 can receive power from a commercial power supply.Alternatively, the lighting device 5100 can use power stored in thepower storage device 5103. Thus, the lighting device 5100 can beoperated with the use of the power storage device 5103 according to anembodiment of the present invention as an uninterruptible power supplyeven when power cannot be supplied from a commercial power supplybecause of power failure or the like.

Note that although the installation lighting device 5100 provided in theceiling 5104 is shown in FIG. 23 as an example, the power storage deviceaccording to an embodiment of the present invention can be used in aninstallation lighting device provided in, for example, a wall 5105, afloor 5106, a window 5107, or the like other than the ceiling 5104.Alternatively, the power storage device can be used in a tabletoplighting device and the like.

As the light source 5102, an artificial light source which provideslight artificially by using power can be used. Specifically, a dischargelamp such as an incandescent lamp or a fluorescent lamp, and alight-emitting element such as an LED or an organic EL element are givenas examples of the artificial light source.

In FIG. 23, an air conditioner including an indoor unit 5200 and anoutdoor unit 5204 is an example of an electric device including a powerstorage device 5203 according to an embodiment of the present invention.Specifically, the indoor unit 5200 includes a housing 5201, aventilation duct 5202, the power storage device 5203, and the like.Although FIG. 23 illustrates the case where the power storage device5203 is provided in the indoor unit 5200, the power storage device 5203may be provided in the outdoor unit 5204. Further alternatively, thepower storage devices 5203 may be provided in both the indoor unit 5200and the outdoor unit 5204. The air conditioner can receive power from acommercial power supply. Alternatively, the air conditioner can usepower stored in the power storage device 5203. Specifically, in the casewhere the power storage devices 5203 are provided in both the indoorunit 5200 and the outdoor unit 5204, the air conditioner can be operatedwith the use of the power storage devices 5203 according to anembodiment of the present invention as an uninterruptible power supplyeven when power cannot be supplied from a commercial power supply due topower failure or the like.

Note that although the separated air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 23 as an example, thepower storage device according to an embodiment of the present inventioncan be used in an air conditioner in which the functions of an indoorunit and an outdoor unit are integrated in one housing.

In FIG. 23, an electric refrigerator-freezer 5300 is an example of anelectric device including a power storage device 5304 according to anembodiment of the present invention. Specifically, the electricrefrigerator-freezer 5300 includes a housing 5301, a door for arefrigerator 5302, a door for a freezer 5303, the power storage device5304, and the like. The power storage device 5304 is provided in thehousing 5301 in FIG. 23. The electric refrigerator-freezer 5300 canreceive power from a commercial power supply or can use power stored inthe power storage device 5304. Thus, the electric refrigerator-freezer5300 can be operated with the use of the power storage device 5304according to an embodiment of the present invention as anuninterruptible power supply even when power cannot be supplied from acommercial power supply because of power failure or the like.

Note that among the electric devices described above, a high-frequencyheating apparatus such as a microwave and an electric device such as anelectric rice cooker require high power in a short time. The tripping ofa circuit breaker of a commercial power supply in use of electricdevices can be prevented by using the power storage device according toan embodiment of the present invention as an auxiliary power supply forsupplying power which cannot be supplied enough by a commercial powersupply.

In addition, in a time period when electric devices are not used,specifically when the proportion of the amount of power which isactually used to the total amount of power which can be supplied by acommercial power supply (such a proportion is referred to as usage rateof power) is low, power can be stored in the power storage device,whereby the usage rate of power can be reduced in a time period when theelectric devices are used. In the case of the electricrefrigerator-freezer 5300, power can be stored in the power storagedevice 5304 at night time when the temperature is low and the door for arefrigerator 5302 and the door for a freezer 5303 are not opened andclosed. The power storage device 5304 is used as an auxiliary powersupply in daytime when the temperature is high and the door for arefrigerator 5302 and the door for a freezer 5303 are opened and closed;thus, the usage rate of power in daytime can be reduced.

Next, a portable information terminal which is an example of electricdevices will be described with reference to FIGS. 24A to 24C.

FIGS. 24A and 24B illustrate a tablet terminal that can be folded. InFIG. 24A, the tablet terminal is opened, and includes a housing 9630, adisplay portion 9631 a, a display portion 9631 b, a switch 9034 forswitching display modes, a power switch 9035, a switch 9036 forswitching to power-saving mode, a fastener 9033, and an operation switch9038.

Part of the display portion 9631 a can be a touch panel region 9632 aand data can be input when a displayed operation key 9638 is touched.Although a structure in which a half region in the display portion 9631a has only a display function and the other half region has a touchpanel function is shown as an example, the display portion 9631 a is notlimited to the structure. The whole region in the display portion 9631 amay have a touch panel function. For example, the display portion 9631 acan display keyboard buttons in the whole region to be a touch panel,and the display portion 9631 b can be used as a display screen.

Similarly to the display portion 9631 a, part of the display portion9631 b can be a touch panel region 9632 b. A switching button 9639 forshowing/hiding a keyboard of the touch panel is touched with a finger, astylus, or the like, so that keyboard buttons can be displayed on thedisplay portion 9631 b.

Touch input can be performed in the touch panel region 9632 a and thetouch panel region 9632 b at the same time.

The switch 9034 for switching display modes can switch the displaybetween portrait mode, landscape mode, and the like, and betweenmonochrome display and color display, for example. The switch 9036 forswitching to power-saving mode can control display luminance to beoptimal in accordance with the amount of external light in use of thetablet terminal which is detected by an optical sensor incorporated inthe tablet terminal. Another detection device including a sensor fordetecting inclination, such as a gyroscope or an acceleration sensor,may be incorporated in the tablet terminal, in addition to the opticalsensor.

Note that FIG. 24A shows an example in which the display portion 9631 aand the display portion 9631 b have the same display area; however,without limitation thereon, one of the display portions may be differentfrom the other display portion in size and display quality. For example,one display panel may be capable of higher-definition display than theother display panel.

The tablet terminal is closed in FIG. 24B. The tablet terminal includesthe housing 9630, a solar cell 9633, a charge/discharge control circuit9634, a battery 9635, and a DCDC converter 9636. In FIG. 24B, astructure including the battery 9635 and the DCDC converter 9636 isillustrated as an example of the charge/discharge control circuit 9634.The power storage device described in any of the above embodiments isused as the battery 9635.

Since the tablet terminal can be folded, the housing 9630 can be closedwhen the tablet terminal is not used. As a result, the display portion9631 a and the display portion 9631 b can be protected; thus, a tabletterminal which has excellent durability and excellent reliability interms of long-term use can be provided.

In addition, the tablet terminal illustrated in FIGS. 24A and 24B canhave a function of displaying a variety of kinds of data (e.g., a stillimage, a moving image, and a text image), a function of displaying acalendar, a date, the time, or the like on the display portion, atouch-input function of operating or editing the data displayed on thedisplay portion by touch input, a function of controlling processing bya variety of kinds of software (programs), and the like.

The solar cell 9633 provided on a surface of the tablet terminal cansupply power to the touch panel, the display portion, a video signalprocessing portion, or the like. Note that a structure in which thesolar cell 9633 is provided on one or two surfaces of the housing 9630is preferable to charge the battery 9635 efficiently. When the powerstorage device described in any of the above embodiments is used as thebattery 9635, there is an advantage of downsizing or the like.

The structure and the operation of the charge/discharge control circuit9634 illustrated in FIG. 24B are described with reference to a blockdiagram in FIG. 24C. The solar cell 9633, the battery 9635, the DCDCconverter 9636, a converter 9637, switches SW1 to SW3, and the displayportion 9631 are shown in FIG. 24C, and the battery 9635, the DCDCconverter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge/discharge control circuit 9634 in FIG. 24B.

First, an example of the operation in the case where power is generatedby the solar cell 9633 using external light is described. The voltage ofpower generated by the solar cell is raised or lowered by the DCDCconverter 9636 so that the power has a voltage for charging the battery9635. Then, when the power from the solar cell 9633 is used for theoperation of the display portion 9631, the switch SW1 is turned on andthe voltage of the power is raised or lowered by the converter 9637 soas to be a voltage needed for the display portion 9631. In addition,when display on the display portion 9631 is not performed, the switchSW1 is turned off and the switch SW2 is turned on so that charge of thebattery 9635 may be performed.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, without limitation thereon, the battery 9635may be charged using another power generation means such as apiezoelectric element or a thermoelectric conversion element (Peltierelement). For example, the battery 9635 may be charged with anon-contact power transmission module which is capable of charging bytransmitting and receiving power by wireless (without contact), oranother charging means may be used in combination.

It is needless to say that an embodiment of the present invention is notlimited to the electric device illustrated in FIGS. 24A to 24C as longas the power storage device described in any of the above embodiments isincluded.

This embodiment can be implemented by being combined as appropriate withany of the above-described embodiments.

EXPLANATION OF REFERENCE

200: silicon substrate, 202: active material, 202 a: common portion, 202b: protrusion, 202 c: protrusion, 204: graphene, 206: negativeelectrode, 206 a: negative electrode, 206 b: negative electrode, 207:protective layer, 208 a: mask, 208 e: mask, 209: spacer, 205: dashedline, 209 a: spacer, 209 b: spacer, 209 c: spacer, 210: silicon layer,211: current collector, 212: active material, 212 a: common portion, 212b: protrusion, 214: graphene, 215: active material layer, 215 a: activematerial layer, 215 b: active material layer, 215 c: active materiallayer, 216: negative electrode, 217: protective layer, 219: spacer, 221:cylindrical shape, 222: conical shape, 223: plate-like shape, 231: axis,233: interface, 241: axis, 251: axis, 260: silicon substrate, 261:active material, 262: active material, 262 a: common portion, 262 b:protrusion, 262 c: protrusion, 264: graphene, 266: negative electrode,266 a: negative electrode, 266 b: negative electrode, 267: protectivelayer, 268 a: mask, 268 e: mask, 268 f: mask, 268 j: mask, 269: dashedline, 270: silicon layer, 271: current collector, 272: active material,272 a: common portion, 272 b: protrusion, 274: graphene, 275: activematerial layer, 275 a: active material layer, 275 b: active materiallayer, 275 c: active material layer, 276: negative electrode, 277:protective layer, 280: active material, 281: cylindrical shape, 282:conical shape, 283: plate-like shape, 307: positive electrode currentcollector, 309: positive electrode active material layer, 311: positiveelectrode, 321: positive electrode active material, 323: graphene, 331:spacer, 400: lithium-ion secondary battery, 401: positive electrodecurrent collector, 403: positive electrode active material layer, 405:positive electrode, 407: negative electrode current collector, 409:negative electrode active material layer, 411: negative electrode, 413:separator, 415; electrolyte, 417: external terminal, 419: externalterminal, 421: gasket, 5000: display device, 5001: housing, 5002:display portion, 5003: speaker portion, 5004: power storage device,5100: lighting device, 5101: housing, 5102: light source, 5103: powerstorage device, 5104: ceiling, 5105: wall, 5106: floor, 5107: window,5200: indoor unit, 5201: housing, 5202: ventilation duct, 5203: powerstorage device, 5204: outdoor unit, 5300: electric refrigerator-freezer,5301: housing, 5302: door for refrigerator, 5303: door for freezer,5304: power storage device, 9033: fastener, 9034: switch, 9035: powerswitch, 9036: switch, 9038: operation switch, 9630: housing, 9631:display portion, 9631 a: display portion, 9631 b: display portion, 9632a: region, 9632 b: region, 9633: solar cell, 9634: charge/dischargecontrol circuit, 9635: battery, 9636: DCDC converter, 9637: converter,9638: operation key, 9639: button

This application is based on Japanese Patent Application serial no.2011-203579 filed with Japan Patent Office on Sep. 16, 2011, JapanesePatent Application serial no. 2011-207692 filed with Japan Patent Officeon Sep. 22, 2011, and Japanese Patent Application serial no. 2011-217646filed with Japan Patent Office on Sep. 30, 2011, the entire contents ofwhich are hereby incorporated by reference.

1. A power storage device comprising a negative electrode, the negativeelectrode comprising: a common portion; a plurality of protrusionsprotruding from the common portion; and a layer including a grapheneprovided over the common portion and the plurality of protrusions. 2.The power storage device according to claim 1, wherein axes of theplurality of protrusions are oriented in a same direction.
 3. The powerstorage device according to claim 1, wherein in a lateralcross-sectional shape, an area of a bottom portion of each of theplurality of protrusions which is in contact with the common portion islarger than an area of a top portion of each of the plurality ofprotrusions.
 4. The power storage device according to claim 1, whereinin a longitudinal cross-sectional shape, a width of a bottom portion ofeach of the plurality of protrusions which is in contact with the commonportion is larger than a width of a top portion of each of the pluralityof protrusions.
 5. The power storage device according to claim 1,wherein the common portion or the plurality of protrusions comprisesilicon.
 6. The power storage device according to claim 1, wherein theplurality of protrusions each have a columnar shape, a conical orpyramidal shape, a plate-like shape, or a pipe-like shape.
 7. The powerstorage device according to claim 1, further comprising a protectivelayer between a top portion of each of the plurality of protrusions andthe graphene.
 8. The power storage device according to claim 1, furthercomprising an insulating layer provided over the graphene of thenegative electrode.
 9. A power storage device comprising a negativeelectrode, the negative electrode comprising: a current collector; acommon portion provided over the current collector; a plurality ofprotrusions protruding from the common portion; and a layer including agraphene provided over the common portion and the plurality ofprotrusions.
 10. The power storage device according to claim 9, whereinaxes of the plurality of protrusions are oriented in a same direction.11. The power storage device according to claim 9, wherein in a lateralcross-sectional shape, an area of a bottom portion of each of theplurality of protrusions which is in contact with the common portion islarger than an area of a top portion of each of the plurality ofprotrusions.
 12. The power storage device according to claim 9, whereinin a longitudinal cross-sectional shape, a width of a bottom portion ofeach of the plurality of protrusions which is in contact with the commonportion is larger than a width of a top portion of each of the pluralityof protrusions.
 13. The power storage device according to claim 9,wherein the common portion or the plurality of protrusions comprisesilicon.
 14. The power storage device according to claim 9, wherein theplurality of protrusions each have a columnar shape, a conical orpyramidal shape, a plate-like shape, or a pipe-like shape.
 15. The powerstorage device according to claim 9, further comprising a protectivelayer between a top portion of each of the plurality of protrusions andthe graphene.
 16. The power storage device according to claim 9, furthercomprising an insulating layer provided over the graphene of thenegative electrode.
 17. A power storage device comprising a negativeelectrode, the negative electrode comprising: a current collector; aplurality of protrusions provided over the current collector; and alayer including a graphene provided over the current collector and theplurality of protrusions.
 18. The power storage device according toclaim 17, wherein axes of the plurality of protrusions are oriented in asame direction.
 19. The power storage device according to claim 17,wherein in a lateral cross-sectional shape, an area of a bottom portionof each of the plurality of protrusions which is in contact with thecurrent collector is larger than an area of a top portion of each of theplurality of protrusions.
 20. The power storage device according toclaim 17, wherein in a longitudinal cross-sectional shape, a width of abottom portion of each of the plurality of protrusions which is incontact with the current collector is larger than a width of a topportion of each of the plurality of protrusions.
 21. The power storagedevice according to claim 17, wherein the plurality of protrusions eachhave a columnar shape, a conical or pyramidal shape, a plate-like shape,or a pipe-like shape.
 22. The power storage device according to claim17, further comprising a protective layer between a top portion of eachof the plurality of protrusions and the graphene.
 23. The power storagedevice according to claim 17, further comprising an insulating layerprovided over the graphene of the negative electrode.